I am interested in others experiences on this issue. Since it came up on page 3 of the Idooski & Co. Project Mxz 800, Thumbdoctor's Bluprinting (http://www.snowmobileworld.com/forum/index.php?showtopic=23320&st=30) pinned thread above I thought a new thread would keep the other thread on blueprinting matters.

The 800 Rotax has an issue where the auxiliary exhaust ports or triple ports are partially exposed on the underside when the piston reaches near TDC. This happens in the corners of the front/exhaust-side skirt cutout. Thumbdoctor pointed this out in his blueprint thread above. Here is a photo from that thread: http://www.snowmobileworld.com/forum/uploads/post-65-1061270500.jpg
This first widely came up before last season when Straightline Performance started offering a piston welding modification which eliminates the overlap:
Straightline Modified Elko Piston (http://www.straightlineperformance.com/images/product/skidoo/skidoopistonmod.jpg)
Many two stroke builders came to the same conclusion as Thumbdoctor and decided this wasn't an issue. At the same time, those running the modification swear by it and with no other changes run greatly reduced jetting. If forum members are running this mod. please post your results.

Around the time this piston mod was first offered I had my 800 away being ported by MacDizzy. Since the jugs were going to be re-nicasiled after porting anyway he offered to eliminate the overlap by reshaping the triple-ports. He wasn't convinced it is a problem, but this eliminated any doubt without chancing a modified piston. Because I had issues during the second season with with the stock piston/ring combination on my Summit 800 I had decided to try an alternative piston. At the time of the porting the only option was the Wiseco. I know they can be run reliably, but I prefer to ride a lot and not have to worry about reliability. I ended up running a cast piston instead.

I had some problems with my engine after rebuild that I still have not been able to figure out. Seeing Thumbdoctor's mention of the cross-circuiting with the piston he shows in his post: http://www.snowmobileworld.com/forum/uploads/post-65-1061271377.jpg makes me wonder if I may have experienced similar problems? I don't recognize the piston in Thumbdoctor's post and didn't realize a piston of that style was available for the 800. It looks similar to the 670 Rotax piston, but I think they only have the piston pin cutout on the intake side. H20XER, idooski and pete all weighed in with answers to Thumdoctor's question. Thumbdoctor or idooski can one of you please provide more information about what you experienced and what you think happened?

Thanks,

SummitAK

If you don't mind, I will give this a try. I am sure that idooski and the doc will also reply.
Both idooski and myself were running Art Pro-x pistons in our 800 Doo's last year. These pistons were not a direst drop in for the Doo motor. They had to have the dome of the piston cut and some skirt work done I believe to get the to rotate in the Doo motor. The stock head would not work either with the pistons do to the fact that the dome was taller and the angle of the squish area was much steeper. We both got custom inserts for the RKT head which brought our compression ratio up to 13 to 1.
On my sled I experienced a loss of top end power...95 mph top speed! Idooski experienced the same thing. Both sleds pulled extremely hard and then fell off just as extreme. After beating our heads to death and spending most of our winter clutching we did not gain any mph. At that point , idooski installed a set of race shop twins and with in a half an hour of run time burned down. His plugs were a tad on the rich side as well as his wash when he burned down. He had the cylinders sleaved and put it back together....burned down again, just as quick!
During this summer idooski noticed that the impression around the wrist pins would allow a major short circuit. This short circuit was extremely pronounced with the twin pipes. It , as best we can tell, was stuffing the crankcase full of spent gasses and and diluting the fresh intake charge to the point of it being lean. I believe at on point he pulled the jets out of the carbs and ran the sled!!
Anyway, after finding this out from idooski, I pulled mine down to inspect. I found that I was experiencing the same spent gas charge as idooski but a lot less extreme. In this pick I only have about 400 miles on the pistons. They show a lot of wear just in front of the short circuit, and a score line down the piston. If you will also notice a lot of carbon build-up in that little area around the wrist pin. These pistons are coming out and for now, the stockers are going back in with the new rings.
BTW, idooski or myself could never get our egt's much over 1000 degrees. A wide open pull at 1050 degrees then shut down would produce white plugs and no wash. If you went out and rode the trails on and off the throttle the plugs and wash would be perfect!

If you are only trying to get better fuel mileage then the modified piston is the way to go.On an 02 mxzx800 you could run down to a 420 or ten jet sizes safelly.My buddie tried the welded piston thing and showed only 1 or 2 hp on the dyno ,but lost ground in a drag race compared to stock pistons.Is there a chance that rotax could have short circuited the port to maybe cool the piston dome(these are big diameter pistons)maybe they engineered it that way for a reason!!!Anyway we took the pistons back out as the jetting became very temp sensitive wiht this set up.Eventually the clyinders were sent to Don Emery for one of his port,reed,kits,and now the sled really rocks.If you are interested in trying the pistons I can get you a deal on a set with only 200kms.

I believe one area which hasn't been brought into this discussion is "what's the exhaust port modifier doing during the worst part of this back pressure dilution" (short circuit), I like to call it cycle sharing. When you look at the port layout on this design you will notice that the shuttle (outside) exhaust ports remain open when the RAVE guillotine is in the closed (lower port roof) position. The exhaust signal to the RAVE using a y-pipe / single pipe remains quite constant so valve operation is as intended. Install twin pipes, now we've altered both scavenging and signal to the RAVEs (the y-pipe inadvertently links the two RAVEs on a single system). No one has stated whether they are sure if the valves are opening 100% at desired rpm. This would explain some crazy EGT readings and poor WOT piston wash. The closed RAVE is essentially shielding the probes. If they remain closed or don't open fully, the exhaust pressure will seek the path(s) of least resistance, end of pipe or reversion back to the side ports where there is low pressure provided by spent crankcase compression. Operating the machine in "Trail Mode" assures correct RAVE operation as you are building pressure in small events and retaining pipe heat as compared to a "lake Run" where port signal pressure may drop as cooling air chills the pipes. This in part explains a quick burn down with twin pipes and larger short circuit provided by wrist pin reliefs. This is an area that more testing is mandated. Unfortunately, achieving "real world" testing in a dyno facility will not provide enough data to really point to a smoking gun.

The piston photoed is that of a taiwanese Knock-off, Kimpex I believe, I pulled them at the Bombi dealer from a traded in '02 machine. I guess you know why!!

Thumbdoctor,

Are you saying that piston pictured is a used one??? It appears to be new.

I have just purchased a set of wisecos. Took some measurements and set it up like the stocker and the short circuit is still there.

Wiseco did add a little stock to the radius but only about .040. looks like there needs to be about .075 or so to cover the port.

Anyhow I'm really wondering if Rotax did this for a reason. These guys can't be that dumb where they would have missed such an issue like this.

This talk is now going on 2 years and I still have not seen a reasonable answer or data to this issue. or maybe it isn't an issue...

The piston boss short looks to be a problem as confirmed by Idooski and MXZWFO.

QUOTE]"what's the exhaust port modifier doing during the worst part of this back pressure dilution" (short circuit), I like to call it cycle sharing. [/QUOTE]

I like that term, Doc. Accurately descriptive. MXZWFO has described the problem quite well. I didn't have any problems with the single pipe, but then I only had it on for a few miles. I put the twins on and overdid the inspection stops. I did about 8 short blasts with an inspection showing borderline lean every time. Jetting up each time seemed to make little difference. I had jetted to 680 and still not enough fuel (or so it seemed)Then I pulled the main jet completely and experienced the same result. I then pulled the needle and seat and drilled out the seat to .090". I put the 680's back in and everything looked very good except the EGT's. During this whole testing period, it was rare to see the temps above 1100 degrees F. That is what was so confusing. I also blew 3 base gaskets during all this. Any one have any ideas about that? I even had someone else build the engine the last time because I was losing confidence. I actually felt better when the exact same thing happened again. At WOT everything seemed to be just fine. We were up at Lake Gogebic and we were doing some racing on the ice. No one could touch me. Things were finally starting to feel good. We did about a 40 MPH run back to the lodge to get the girls and it seized up, ripping up all the nicasil. First day of vacation. :cussing: :cussing: I didn't realize the wrist pin recess short circuit until this summer. I feel that this is the main problem that I have been fighting all along. I don't believe that the little short circuit past the bottom of the piston is a problem. There have been a lot of engines with that same short over the years with no apparent problems.

The exhaust signal to the RAVE using a y-pipe / single pipe remains quite constant so valve operation is as intended. Install twin pipes, now we've altered both scavenging and signal to the RAVEs (the y-pipe inadvertently links the two RAVEs on a single system). No one has stated whether they are sure if the valves are opening 100% at desired rpm.

Are you suggesting that we need to run a lighter spring when running twin pipes? The new HO cylinders have a raised boss on them where the RAVE assy's are mounted. I don't know if the RAVE assy itself is any different though. This would allow the valves to open earlier, would it not? Or is it just to get the valve back in sync with the higher exhaust port timing?

This will be squared away this winter! :thumbsup:

My $.02,FWIW. The rave spring tension can only be tuned by testing. On wfo dyno runs the best setting has them pop open around 6000. There is usually several clicks of the adjuster tolerance so when testing use full or half turns. You can make holes in the top of the caps to see them move. This CANT be done on a track stand, wfo under load only. If the valves open too soon or too late hp under the curve will drop. This should give the best acceleration. Compression mods, porting, carb size, altitude, etc can change the pop open rpm so there is no generic setting for all sleds.

The problem, and why I'm a believer and postive control of exhaust valves (not springs). Do a long dyno wfo 30sec pull, pipe heats up, scavenging changes, twist the rave adjusters around and you can see better #'s.











.02 on the short circut, raising the cyl helps, as does stiff reeds. I don't have the balls to weld on the piston, welding on the ports might be better but reduction in flow would negate the gain?

02 on the short circut, raising the cyl helps, as does stiff reeds. I don't have the balls to weld on the piston, welding on the ports might be better but reduction in flow would negate the gain?

I honestly don't believe that short circuit under the piston is the problem.

The one around the wrist pin is.

Originally posted by idooski@Aug 22 2003, 03:30 PM

I honestly don't believe that short circuit under the piston is the problem.

The one around the wrist pin is.
I agree, the piston seems to be a worthy suspect. As for the RAVE valve. I'd be willing to engineer an electrical indicator(idiot light using the high bean bulb on the instrument panel) utilizing a housing mounted micro switch to assure positive valve opening. Another approach I'd like to explore is to tap into the signal circuit for the RAVE and install a variable bleed rate check valve to hold the guillotine open when there is small pressure changes at WOT.

Hey guys thanks for the valuable information on your experiences.

mxzwfo, I am running almost the same set up as you and idooski except that the "short circuit" below the piston has been closed and the port reshaped. My problems may also be compression related too. What was the uccr for the domes you guys ran? I'm assuming you were near sea level?

John, thanks for the piston info. Not many that opted for the welded pistons have posted results whether + or -. Thanks for the offer, but with my jugs modified the pistons aren't a necessary option and I was scared to try them anyway. My intake porting is similar to what Don is providing, but my exhaust looks to be reshaped quite a bit compared to the DNE examples I've seen.

Thumbdoctor, that is exactly the area I wanted to hear more about. I guess a lot of it is theory without testing, but this type of testing seems beyond the capabilities of even the high end mod businesses. What you mention makes a lot of sense. I've always like the simplicity of the RAVE system, but at the same time there seems to be no easy way to tune them as Hillpounder mentions. I think Russ Lemke said very early on with the new 800 that he thought the exhaust valves crept closed on long runs. Haven't heard as much about this recently.

idooski, if you have photos of your seized piston I am interested in comparing. I have some I can send you too. I too want to get this straigtened out this winter. I am going to a Summit REV and would like to make use of my extra ported jugs, but I'm not sure about such things as the HO timing for the raised exhaust port and the modified RAVE's you mention too.

Thanks,

SummitAK

Have any of you guys with the RKT piston kit compared the amount(size of gap) of the short-circuiting to the stock amount? I know and have ridden with a friend with one of these kits and it gave him fits all season trying to make decent power with it. I have heard that the gap is larger with the RKT kit??? Those pistons are also heavier than the stockers, not sure what that would do to top end power.

So you guys are thinking that the short-circuiting is happening as the circlip is over the aux exhaust port? Isn't the piston sealing exchange with the transfer ports at this point in time?

I wasn't going to join over here but this tech section is excellent, well done.

I had Don port my cylinders which requires an 8 hole base gasket which raises my ports .008 from the stock 6 hole. I also have the RK-TEK head with the domes modified because kelsey suggests the 6 hole for the sea level domes, i am at 165 for compression. I also have the WISECOS that have ceramic coated domes and moly coated skirts. I am running twin crank shops with taper bored carbs to 42mm. My needle jet is bored out .004 with a 600ho jet needle.I modified the reed cages and i am using boysen's with reed spacers. I am running 600 mains with 25 pilots. So far so good, I dont have alot of time on this setup but so far it seems to rock the house. Is there anything that i should be watching considering you guys seem to have alot of test time with alot of different setups in this area. My egt's are good and my plugs are perfect, wash is non exsistent on this twin. SWRules

Originally posted by Thumbdoctor+Aug 22 2003, 05:29 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Thumbdoctor @ Aug 22 2003, 05:29 PM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-idooski@Aug 22 2003, 03:30 PM

I honestly don&#39;t believe that short circuit under the piston is the problem.

The one around the wrist pin is.
I agree, the piston seems to be a worthy suspect. As for the RAVE valve. I&#39;d be willing to engineer an electrical indicator(idiot light using the high bean bulb on the instrument panel) utilizing a housing mounted micro switch to assure positive valve opening. Another approach I&#39;d like to explore is to tap into the signal circuit for the RAVE and install a variable bleed rate check valve to hold the guillotine open when there is small pressure changes at WOT. [/b][/quote]
You&#39;re going to have as much time in this machine as I am. If you want to design anything for the RAVE system, I&#39;ll build it and test it for you.

mxzwfo, I am running almost the same set up as you and idooski except that the "short circuit" below the piston has been closed and the port reshaped

Did you have the port welded up to get rid of the short circuit?

dooski, if you have photos of your seized piston I am interested in comparing.

I sent some to you.

Originally posted by canucklehead@Aug 22 2003, 11:16 PM
Have any of you guys with the RKT piston kit compared the amount(size of gap) of the short-circuiting to the stock amount? I know and have ridden with a friend with one of these kits and it gave him fits all season trying to make decent power with it. I have heard that the gap is larger with the RKT kit??? Those pistons are also heavier than the stockers, not sure what that would do to top end power.

So you guys are thinking that the short-circuiting is happening as the circlip is over the aux exhaust port? Isn&#39;t the piston sealing exchange with the transfer ports at this point in time?

I wasn&#39;t going to join over here but this tech section is excellent, well done.
The TDC short circuit seems to be about the same if not slightly less with the ART pistons.

I come up with the ART piston being almost an ounce lighter than the ELKO.

Originally posted by idooski@Aug 25 2003, 01:42 PM

You&#39;re going to have as much time in this machine as I am.

If you want to design anything for the RAVE system, I&#39;ll build it and test it for you.
I better not! . I have my own problems & you should be spending more seat time and less hair pulling!. We&#39;ll get her fixed even if I have to come down to Kalamazoo and show that primadonna mill who&#39;s boss.

On a serious note once you have the engine near built send me a RAVE assembly you don&#39;t need in a hurry and I&#39;ll build a prototype with all the bells and whistles. Then you can patent & mass produce it, I don&#39;t need to give the IRS any more!!!.

Idooski, Thanks for the photos. I received them this morning.

Did you have the port welded up to get rid of the short circuit?


Yes I did have the port welded up. I sent you some photos that show the change.

I come up with the ART piston being almost an ounce lighter than the ELKO.


I found the same thing when I measured mine compared with the stockers.

SummitAK

Originally posted by speed is everything@Aug 25 2003, 02:52 AM
I had Don port my cylinders which requires an 8 hole base gasket which raises my ports .008 from the stock 6 hole. I also have the RK-TEK head with the domes modified because kelsey suggests the 6 hole for the sea level domes, i am at 165 for compression. I also have the WISECOS that have ceramic coated domes and moly coated skirts. I am running twin crank shops with taper bored carbs to 42mm. My needle jet is bored out .004 with a 600ho jet needle.I modified the reed cages and i am using boysen&#39;s with reed spacers. I am running 600 mains with 25 pilots. So far so good, I dont have alot of time on this setup but so far it seems to rock the house. Is there anything that i should be watching considering you guys seem to have alot of test time with alot of different setups in this area. My egt&#39;s are good and my plugs are perfect, wash is non exsistent on this twin. SWRules
Well any suggestions !!

Originally posted by speed is everything@Aug 25 2003, 04:49 PM
Well any suggestions !!
Didn&#39;t know you were looking for any. Looks like you got her running the way we would like to have ours running.

Do you have a specific question? Sorry we didn&#39;t catch it.

Keith

I noticed that the profile on the Wiseco piston has about .050 more material. That and a .8mm gasket on my machine and I have no short circuit.

That short circuit there doesn&#39;t concern me Dave. I don&#39;t believe that there is reason for bother over that TDC short circuit.

Originally posted by Thumbdoctor+Aug 25 2003, 03:38 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Thumbdoctor @ Aug 25 2003, 03:38 PM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-idooski@Aug 25 2003, 01:42 PM

You&#39;re going to have as much time in this machine as I am.

If you want to design anything for the RAVE system, I&#39;ll build it and test it for you.
I better not! . I have my own problems & you should be spending more seat time and less hair pulling!. We&#39;ll get her fixed even if I have to come down to Kalamazoo and show that primadonna mill who&#39;s boss.

On a serious note once you have the engine near built send me a RAVE assembly you don&#39;t need in a hurry and I&#39;ll build a prototype with all the bells and whistles. Then you can patent & mass produce it, I don&#39;t need to give the IRS any more!!!. [/b][/quote]
I just meant with all the porting and all that you will have a great deal of tiime in it :p

And don&#39;t think for a second that we all don&#39;t appreciate it. I get e-mails on it every day as I know you do. Keep up the good work man. Everyone likes it. :thumbsup:

Interesting, I was told that it was a watercraft piston and it was heavier. :doh:

I&#39;m sure you guys have read Kelsey&#39;s very long reply on snowest claiming that it is not a performance upgrade but rather a reliability upgrade.

I know that there is a ring issue, I burned down on my third ride of the year on a brand new motor, rings already were flaking. I need to tear down before this season and expect to at least replace the rings. :cussing:

Originally posted by canucklehead@Aug 27 2003, 11:31 PM
I&#39;m sure you guys have read Kelsey&#39;s very long reply on snowest claiming that it is not a performance upgrade but rather a reliability upgrade.

I know that there is a ring issue, I burned down on my third ride of the year on a brand new motor, rings already were flaking. I need to tear down before this season and expect to at least replace the rings. :cussing:
Given the choice, I take reliability over peak performance any day. The area I&#39;ve explored in ring failures is not related to head design, not related to compression ratio and not related to combustion heat. What I&#39;ve found is shoddy chamfering done on the exhaust port at R*t*x. careful detail in this area helps greatly. Ask yourself, why do some series III stay on the up &#39;n up while others experience ring failure time and time again. Don&#39;t always blame the usual suspects :D

Given the choice, I take reliability over peak performance any day.

Me TOO! but sometimes you can have both and that makes for a great Winter.


TD, I have to agree with you on the chamfer also. It may be a big part of the overal ring saga. Nice snag!!


Kelsey

TD- I use the Summit rave springs in the flat lander MXZ&#39;s ORRR just turn the rave caps out flush- then there&#39;s no issue with the raves dooing anything other than what there supposed to doo.

speed is everything- I would spend the time/money to dyno that package and tune it in before ya hit the snow- also for clutching get a set of heavy adj opins from Goodwin and start at 20 grams(load that baby down) and work your way around from there-BJ

Originally posted by BIG JOHN@Aug 30 2003, 11:54 PM
speed is everything- I would spend the time/money to dyno that package and tune it in before ya hit the snow- also for clutching get a set of heavy adj opins from Goodwin and start at 20 grams(load that baby down) and work your way around from there-BJ
Try and find someone in your area with a flywheel dyno (Inertia Dyno) so that your not spending money to find out peak power at a given RPM. What an inertia dyno measures more accurately is power under the curve, engine burst acceleration. Let&#39;s call it "Quality HP" instead of how much "Quantity". The real world will be repeatable #s. consistency out there in the snow.

Originally posted by SummitAK@Aug 21 2003, 06:15 PM
I had issues during the second season with with the stock piston/ring combination on my Summit 800 I had decided to try an alternative piston.
:hallo1: I spoke at length with an acquaintance of mine who builds Mercury f1 motors. They have had similar issues with rings dropping the moly filler on the exhaust side too. I learned that they were plating their 3.4 liter piston rings with TiN : Titanium Nitride (Idooski will tell you lots about this stuff) See attached photos.

Another:

As you can gather from the photos, Mercury&#39;s rings are quite similar in design to Rotax. They also employ a 7 degree angle on the upper thrust face to effectively push the ring outward against the cylinder as combustion pressure forces the piston downward in it&#39;s stroke.

One thing to note about this design is the ring is mechanically forced against the cylinder wall throughout the entire downward stroke by the piston ring land interface (ramp) and doesn&#39;t let up until the piston reaches BDC and changes direction. Since the bottom thrust face of the ring is square, the only force applied outward to the cylinder wall on the upward stroke is the ring&#39;s own spring tension and cylinder compression. On older rotax designs which used Dykes type (L-shaped) rings, sealing pressure was applied directly to the upper exposed surface of the ring which carried it downward until the exhaust port opened thus relieving the combustion pressure on the ring reducing thrust against the lower exhaust port chamfer. The results of this design was easier life on the ring as it only had to endure high pressure on the combustion (downward stroke) as it reached the upper chamfer of the exhaust port and it&#39;s own tension across the lower exhaust port chamfer. Multiple ring designs aid this situation too, by applying combustion pressure to 2 low tension rings, you are effectively passing the same pressure over more surface area thus applying less force at the chamfer.

Now, where am I going with all this ?. The fact that the stock piston / ring combination doesn&#39;t have an easy life, attention needs to be paid to the exhaust port chamfer. I have noticed a lot of bombardier / Rotax series III engines with poorly ground or concentrically ground chamfers. I spent a lot of time looking at ring failures on the exhaust port area on Mercury F1 engines and they all point at the same issue: Port Chamfer &#33;!! . The radius of the chamfer required to gently ease the ring back into the piston&#39;s ring land (ring groove) should be broad in the center and reduced toward each side. Consider it a smile on the lower side of the port and a frown on the upper side. Lower side being more important due to the angle the exhaust port departure. This chamfer should blend into the exhaust port and not have any sharp progression. Isometrically angled chamfers tend to cause ring flutter when the ring passes over the exhaust port opening. The ring distorts and tries to enter the port abruptly with with catastrophic results. This flutter (hammering) will cause the ring to twist horizontally and I suspect the moly filler which is softer than the ring&#39;s rail material suffers. After correct chamfering is performed, the area around the chamfer should be worked with a 320 grit flap wheel at low RPM(150).

I find with the combination of plating / aluminum in the exhaust ports, the best way to correctly radius the chamfer is to use a 3/4" sanding drum with fine grit (180) emery tube. This can also be done with a high quality rifler file being careful to file into the port from the bore side. Clean up horizontal chamfers with 320 grit flap wheel after. You can also substitute a flap wheel (Quite expensive) for strips of 320 or 400 grit cloth backed emery cloth pulled across the port chamfers.

Finished chamfer is about 2mm at it&#39;s Center (widest point):

Thanks Doc! I was wondering what I should do in that area. I pulled the motor back down to take the Pro-X&#39;s out and put back in the OEM&#39;s .
Is it wise to pay that much attention to the other ports too? Also, one thing I noticed with the stock pistons, the back side has a blue tint to it, like they have been hot...too hot. I checked all of the dimensions and they are as new, with 5-6 thou, cylinder wall clearance, and they are round within a half thou. Should I worry about the color?


Brian

Brian,
It would be wise to check the other ports for anomalies in the chamfers but all the problems I&#39;ve seen are in the big port. Another tip I was given by Mercury is to hone the cylinders with a Sunnen with diamond stone at 180 grit then smooth the high points (intersection of honing lines) down with a 220 grit sponge backed flap wheel. My F1 contact says that this will eliminate any chance of crosshatch point "bending" during ring break in caused by the additional lubricating properties provided using synthetic oil. The coarse crosshatch has been proven by Merc to hold oil better sustaining higher compression readings. A light hone should be performed if you are changing rings for new ones or in your case, putting pistons with different ring placement back in, as the small step in the bore created by the old ring&#39;s excursion may upset (give bumpy ride) to the Rotax single ring piston design. I wouldn&#39;t worry too much about the bluing. Here&#39;s a photo of a brand new series III piston, note the colour.

Doc I agree with the chamfer theory especially on the 600HO where theres a huge port area in such a small diameter bore. However every bad ring Ive seen is showing signs of damage at points on its entire circumference, not just at or near the exhaust port, that applies to both 600 and 800 motors. Another point Id like to make on the ring issue...... Why dont we hear of these problems with the 700? Same rings, same cylinder design? Im still convinced its all about heat and the motors ability to disipate it under heavy load... the people who ride these sleds like grandma dont seem to experiece the issues as frequently, while those who run in the mountains or high speed trails where high load is present suffer multiple failures.

Originally posted by TT670@Sep 1 2003, 10:57 AM
Doc I agree with the chamfer theory especially on the 600HO where theres a huge port area in such a small diameter bore. However every bad ring Ive seen is showing signs of damage at points on its entire circumference, not just at or near the exhaust port, that applies to both 600 and 800 motors. Another point Id like to make on the ring issue...... Why dont we hear of these problems with the 700? Same rings, same cylinder design?* Im still convinced its all about heat and the motors ability to disipate it under heavy load... the people who ride these sleds like grandma dont seem to experiece the issues as frequently, while those who run in the mountains or high speed trails where high load is present suffer multiple failures.
TT670,

Good to here from you !. Most of the rings I&#39;ve seen are in their early stages of damage, I would imagine, left long enough unchecked, you would have damage on the entire ring (from vibration). Have you experienced flaking around the inlet ports ?, if so, this raises some doubt about the high combustion heat relation as that side of the piston / ring assembly is cooled by the incoming fresh charge. Could someone send me an extensively damaged ring. I can have it checked for temper and give some info as to the nature of the failure, heat related or chronic vibration. As for the 700 I too haven&#39;t seen ring issues. Possibly Titanium Nitride plating of the piston ring for extreme riding style is the answer.

Originally posted by TT670@Sep 1 2003, 09:57 AM
Why dont we hear of these problems with the 700? Same rings, same cylinder design?
Kid Rocker had a 2000 MXZ 700 that would toast a set of rings in a 1000 miles or less. The chrome face would come off just like the 800 does. His would get to the point it would not run at all...with NO notice.


Brian

I have often wondered what the connecting rod to stroke ratio was for the 800. The piston and ring problems led me to believe that this motor may have a short rod length for its stroke, causing excessive side loading on the piston. AS far as not hearing it on the 700s, I would say that they did exibit signs of overheating of the rings causing them to loose power on repeated high load runs. Granted not the same ring flaking issue but still a fundamental design issue related to excessive heating of the rings. I can attest to the fact that the 700 is not even close to the same motor when it is equipped with a two ring piston! Just looking at the single rail ring and Its easy to see its asked an awfull alot of. Well maybe Rotax will come up with a new design and we wont all be caused to scratch our heads so much.

The bottom ring of summit X piston are of the same design as the series 3. Running them in 700&#39;s I&#39;ve never seen any failure (never seen any problem in the 670 either) without doing anything to the ports. Same cyl&#39;s produced flaked single rings? If it&#39;s a chamfer problem it would have to be difference in the way ring contacts it, as thumbdoctor describes. The second ring on the X piston is sharing alot of the work (heat and pressure) with the big dykes ring so it lives in alot safer enviroment than the single ring, but they both would be subject to bad port chamfers. Is this possible: the moly material that is sandwiched between 2 thin chrome bands expands and contracts at a different rate than the chrome bands, and the ring just looses it ability to hold the fill? How about this: The moly doesn&#39;t tranfer heat very well to the bore so all the work is left to the thin chrome bands very small surface area, they get so hot that they "flash" the oil film that they should be surfing on? The first 00 700 pistons did not have the grooves under the ring lands, later in the season an 01-0n they came out with update pistons that had the deep "oil retention" grooves. Was this for the pistons? or an attept to save the rings by holding a supply of oil under them? Thumb doc, you mention the finish hone. Back when moly rings first came out for V8&#39;s a machinist who was prepping a race motor for me finished the bore with 1000moly . It was like mirror, smooth as a baby&#39;s butt. The rings never really seated and the the motor used oil but it picked revved like a SOB and went fast (1/4 mile). You could tell the difference when you hit the starter. I&#39;ve always wondered how the same thing would work in a twostroke?

Could someone send me an extensively damaged ring. I can have it checked for temper and give some info as to the nature of the failure, heat related or chronic vibration.

T.D., I think I have one in the garage if you are interested. It was installed with the &#39;01 piston update at the dealer. Attached is a photo showing both pistons from the intake side. The PTO piston shows moly failure near the ring ends. You may be onto something with the vibration angle. Wasn&#39;t Doo/Rotax quoted in SnowTech last year blaming some ring problems on vibration from detonation? Seems like I saw something like this in one issue.

I&#39;ve attached the exhaust side view of the PTO piston to the next post. Interesting to see that the failure areas appear to line up with the outer portion of the main exhaust port.

I don&#39;t have the MAG piston from this set because it seized at the end of the previous season. I had the dealer do the repair because I received partial warranty coverage. Four rides before end of season and then tearing down over the summer to find that the dealer had left the flaking PTO piston ring installed as is:(

Thanks,

SummitAK

Here is the exhaust photo to go with the previous post:

Originally posted by hillpounder@Sep 1 2003, 01:06 PM
1) with the big dykes ring so it lives in alot safer enviroment
* * than the single ring,

2) but they both would be subject to bad port chamfers.

3) Is this possible:* the moly material that is sandwiched
* * between 2 thin chrome bands expands and contracts at a
* * different rate than the chrome bands, and the ring just
* * looses it ability to hold the fill?
Great points Hill Pounder,

This dialogue is getting great !

I&#39;ll try to reiterate my description of Dykes type rings compared to Trapezoid type with a sketch to help my weak previous explanation of combustion gas pressurization.

Sketch:

1) This is a cross section of a Dykes type ring in it&#39;s groove. As you can see, combustion pressure acts directly on the ring through getting behind it and pushing it against the cylinder wall. Some of that pressure pushes the ring down onto the lower thrust face of the ring groove further sealing it. The prevailing outward pushing pressure is reduced as soon as the exhaust port is uncovered as combustion (expansion) exits through the exhaust port leaving the ring&#39;s own spring tension to ride over the lower chamfer in the exhaust port. This means that the lower chamfer is not as critical, as the ring is only applying light spring tension against it. This is a true gas pressurized ring. That is why on previous designs lower port chamfer was so forgiving on rings. You could get away with poor OEM grinding. The other change is that old style cast iron sleeves were a lot softer than plating so the hard ring would make it&#39;s own chamfer profile in the exhaust port. Nikasil is resilient and is very close to the same durometer hardness as the ring creating a catastrophic event when the ring catches the bottom chamfer at high RPMs (My style of riding !).

2) This is a cross section of a Trapezoid style ring in it&#39;s groove, it is not as exposed to combustion pressure as the above Dykes ring. Trapezoids rely on the combustion pressure bearing down on the piston and the ring is mechanically pushed outwards through the 7 degree ramp on the upper thrust surface of the ring and matching ring groove on the piston. This mechanical push is not relieved when the exhaust port is open like a Dykes gas pressurized ring, it pushes outward the entire downward stroke. This is why the lower exhaust port chamfer is more critical than other designs. The ring is trying to push out the exhaust port the the edge on the lower chamfer slams the ring back into the ring groove.

3) The moly filled ring design is actually a u-channel lying on its side with the cavity filled with Molybdenum instead of two rails sandwiching moly in between.

Now hillpounders on to something with the moly filling and chrome rings expanding at different rates, especially when your running it hard and the temps take a rapid climb. Thumbdoc, my 600HO had the moly fail at 1800 miles and it was chipping in the areas of all the ports, but the exhaust area had the worst damage. Its very odd how it chips, almost looks similar to miniscule paint chips on a cars hood or bumper, of course an a much smaller scale. That too kinda makes me think the port chamfer isnt the issue. When I first inspected my HO I could run a dental pick across the face of the ring thru the rave opening and feel the chips. Wish I had a set to send you, I had to turn them in for warranty. I do have some pics I can email.. not sure if the resolution will show what youre looking for. While were on this, in the spring I spent some time looking for a ring to replace the 600Ho ring. The closest I found was a polaris 550 fan ring. They also use a 7 degree semi keystone ring thats ductile iron. Its a 73mm instead of a 72mm ring but I thought maybe with some filing it would work.. any thoughts? anyone wanna be the guniea pig? Im also contacting Total Seal tomorrow, they claim they&#39;ll make rings for anything, if theyre interested and it works out maybe we can organize a group buy.

TT670,

A 73mm ring in a 72mm hole is kind of like the good old days when we bought a complete set of over sized rings and dry fitted them to each hole. Two questions:
1) Is the ring profile the same
2) Is the tension the same

One test I&#39;ve done to more or less prove that ring heat damage is less of a concern than disintegration is the fact that the perceived overheated rings which allegedly could not transfer enough heat the the cylinder walls still had good static tension. We measured them by standing them on end with the end gap parallel to the scale&#39;s plate and compressing them til the end gap closed on a .012" feeler gage. they were very close with a difference of about 4 grams. I then heat cycled a used set to 1300 F and they lost nearly all static tension. One remarkable thing was the moly filler stayed intact.

If the answer to both questions is yes, then try it. you will need to hone the bores anyways. One advantage now with SwainTech is if your skirt goes undersized, you can build it up with teflon and in your case you&#39;ll have enough ring material to compensate. I&#39;m going to find out from Idooski what the costs associated with TiN plating a ring set would be. I inquired with Hastings who manufacture some Merc racing rings under licence and found out their unobtainium rings like the elusive Bombardier new series III racing rings cost $300.00 a copy. Hasings isn&#39;t interested in dealing with small quantities.

I&#39;m going to annoy my dealer friend into finding me some really bad 600HO pistons & rings, then I&#39;ll do another step by step with my notes. I&#39;m eager to find the smoking gun too.

TT, give me the thickness and width of the 72mm rings your after. I have some 72&#39;s that bombi sent me by mistake, not sure what they were for but your welcome to them if they&#39;re right.


Thumbdoc, Nice explanation on the way the rings work. What&#39;s your opinion on the difference in drag between the dykes 2 ring piston and the moly single ring. I&#39;ve always felt my motors revved and made better power (intially) better with one ring? Since the trapz ring is dragging through the whole downstroke, does it smaller surface area make up for that? The U shaped moly ring pic, thanks, sandwiched was not a good description. Just a thought Take ANY piece of metal that is u shaped, tubing, flat stock, sheet metal, I don&#39;t care. Heat it and the U WILL open up. Not sure if it means anything.

On the dykes type piston there is quite a gap at the dome, is this a trap for end gasses? or not a concern? have you ever run without the second ring?

thanks hP

Hillpounder,

On older model Dykes equipped cast pistons you could get away with single ring because on a casting the themal expansion wasn&#39;t too much. Forged (compression extruded) needed a second ring just to protect the piston from self destucting & becoming part of the exhaust system during warm up let&#39;s talk loose !!. Low tension ring were the way to go on cast iron cylinders but nikasil is so hard, low tension rings tend to skate on the bore surface. Just look at the piston scratching occurring on large displacement twins running on plating the stuff is brutal !

The combustion chamber exposed side of Dykes rings needed quite a bit of expansion room, not anything to worry about. These rings under heat stress tend to grow larger at the top than bottom.

Doc, I went out and checked the port chamfer on the exhaust...They were rough!! They are now smooth and shaped like the pic you posted.

Thanks,
Brian

Doc I donthave any other info on the 73mm rings, I dropped my search when I heard ski doo was correcting the issue for 04, but I may order a set just to investigate. Hilpounder, I appreciate the offer but I dont have my motor apart to measure, However I recall laying them on a bench last season they were the same thickness as my 800 rings.. just a smaller diameter.

EDIT::: I just got off the phone with Total Seal, They WILL do rings for us, the 72MM 600HO bore is about as small as they&#39;ll go. They need the pistons and cylinders. The pistons do require a modification to run the rings and they need the cylinders to achieve proper fitment.. Cost $125 per cylinder, approx 10 day turnaround.

TT670,we use the total seals in hi-per car engines,and have always wondered if they could be made to work in a two stroke,question,how could you keep the rings overlap from ending up in the ports???If you used the locating dowel as stock you would think that there would be no way of having the ring ends overlap ,interesting but scarey>

The tech guy I spoke with at Total seal told me theyd be using a 2 piece ring assy and would have to modify the pistons for the rings. Im considering pulling my motor apart, sending the cyls to Don Emery first for porting then send them off to Total seal.. Someones gotta be the Guniea Pig right?

Yeah somebody has to be the first.I saw what some experimental rings did last winter in a 440 mxzx,it wasnt pretty what happens when the ring got hung up in the rave valve.Let me know what the guys at total seal say as i would be interested(only after you have put a few miles on yours)Maybe they are going to slot the rings and put a verticle dowel pin through both rings and ring lands,cant think of any other way to keep them from turning,while allowing them to float.

TT, the way I understand it, FWIW? the gapless rings are actually 2 thin rings stacked. One ring uses the stock locater pin location with a low pin, the second ring uses an added pin located higher in the groove. I&#39;m not sure how the rings get tension with this design, mechanically or from combustion pressure? Try em and let us know! I know h2oxer on one of these forums tried them last year.


Thumbdoc, so are you saying the dykes rings (670HO) don&#39;t have enough tension to run on nicasil? I&#39;ve put alot of them in 700&#39;s?

Although Ive never personally used the 670 piston in a 700 ive raced gainst and ridden a couple that did, and man do they run good. Kelsey at RKT disses them saying they have too much drag and HP is lost...Ive seen the results from more than 1 700mxz getting those piston/rings and everyone of them was a rocket... with no other changes done.

May I chime in here with my first post on these forums? I tried to last night, but it wouldn&#39;t let me post. :( All seems better now. :D

Anywho,

As for the ring issue and especially concerning the chamfer question:

I replaced the pistons in my 02 Summit 8 a yr ago at 1500 miles, mostly all off trail. My rings are not quite as bad as SummitAK&#39;s, but similar. My rings show the largest amount of failure in-line with the solid part of the cyl wall next to (and slightly into) the exh port. The part of the rings that would be in the middle of the port are in OK shape. The PTO side does show some signs of that "paint job type cracking" that was previously mentioned in the middle of the exh port. The ends of my ring are flaking too tho.

I believe these findings to be along Kelseys line of thinking - in that the ring along that area that is in contact with the cyl wall the most (always) is the part that is transfering the most heat out of the piston. ???

I would think that if my motor had chmfr issues it would show up all along the port, and probably mostly in the middle of the port??? Thatbis not what I see here.

These pistons/rings were ran all stock with 480 mains and 942 needles.

Maybe we can get these motors all fingered out ina cpl more yrs? :0:

This is also where SummitAK&#39;s worst wear is too. (Pic of his piston)

mxzwfo-I had the exact same problem with mine last winter. I had the ART pistons, intake mod, and RKT head($700) both with and without shim. I think we pm&#39;d each other last winter on Snowest. I had the same complete loss of top end power-would not do 100 mph. I was told to retard the timing 2 degrees-after which it was lucky to do 85 mph-and then it burnt down. Oh well get another piston and cylinder ($500)-no big deal it is only money. And lost a weekend of riding. Corresponded with Kelsey and he says don&#39;t advance the timing-well I did anyway and with 2 degrees it finally came to life and started to run pretty good. Real awesome for trail riding when you are on and off the throttle. Was out lake racing (and kicking butt) all day and on the way home it stuck on a shorter pull than we were racing. Another $400 for piston and nicasil-at least it stuck at the end of the day! Rebuilt it again and it lasted about 1.5 hours when I pinched it it went down at about 500 feet. Another $400. Total $2000 for pistons, head, intake mods and a few nicasil jobs. Good thing the season was over because I was running out of money. I brought this forum discussion up (by email) to Kelsey and he says I was the only one in the universe who had problems that ran the stock single pipe. All I am trying to do is get somewhat of a deal on RKT domes for Skidoo pistons so I can sell the sled and not feel like I am selling a time bomb. He did send me a free set of domes to try to remove the 2mm shim to see if that would wake the motor up-it just bothers me when I see on here that others had performance issues with this piston kit and he continues to tell me I am the only one! He did try to help but short of shipping him the sled it is hard to diagnose over the internet. The sled ran great when it was stock-the rings were even good when I tore it apart-I just wanted a little more. After I installed the pistons, etc. I had to take weight out of my primary-not a good sign.
Now I get an email from Kelsey telling me he does not appreciate that I called myself a guinea pig for buying his parts! I spent $2000 last year with all of the burndowns and he gets pissed?

rangelec, it may not relate to your problems but last winter had to be the biggest burndown year I have ever seen around here. I went down twice, almost all my buds at least once. This was with proven previous season tested combinations! We all run pump 93 premium and they did something to the frickin fuel. We had a mild winter down low so I suspect they left the vapor pressure low, like summer fuel?
Anyway the solution seemed to be to splash some av, race or solvent in the fuel, though we all swore that off years ago. Also was you 800 an mxz or summit? I ask because it&#39;s may understanding that the summit box had 4 more degrees of timing in it. So if RKT said retard 2 degrees (summit) and you advanced 2 (mxz) and found more power, you&#39;d end up right about where he wanted you?

I have an MXZ-he knew that also. My head was setup for 87 octane and I ran either in it. The last two times it went down it was 40 degrees+ for airtemp. The thing runs real good for trail riding but wide open on a lake something is missing for top end power. This is not my first mod either-done many and clutched many for me and others. Kelsey kept telling me it was my clutching but I put it all back to stock and it would only pull 7400 rpm-the clutch dyno tells me it was less hp than stock-until I advanced the timing. I will try again this winter if I keep it and throw much bigger main jets in it and see if it lives but I was getting real tired of paying the nicasil guy and sending Kelsey $150 + gaskets every other week.

Originally posted by Ox1@Sep 3 2003, 07:33 PM
Anywho,

As for the ring issue and especially concerning the chamfer question:

I replaced the pistons in my 02 Summit 8 a yr ago at 1500 miles, mostly all off trail. My rings are not quite as bad as SummitAK&#39;s, but similar. My rings show the largest amount of failure in-line with the solid part of the cyl wall next to (and slightly into) the exh port. The part of the rings that would be in the middle of the port are in OK shape. The PTO side does show some signs of that "paint job type cracking" that was previously mentioned in the middle of the exh port. The ends of my ring are flaking too tho.

I believe these findings to be along Kelseys line of thinking - in that the ring along that area that is in contact with the cyl wall the most (always) is the part that is transfering the most heat out of the piston. ???

I would think that if my motor had chmfr issues it would show up all along the port, and probably mostly in the middle of the port??? Thatbis not what I see here.

These pistons/rings were ran all stock with 480 mains and 942 needles.

Maybe we can get these motors all fingered out ina cpl more yrs? :0:

This is also where SummitAK&#39;s worst wear is too. (Pic of his piston)
Nice to see you drop in Ox.

As far as the wear location due to the chamfer, I think that the wear is going to be nearer the edge of the port. The ring is extended into the exhaust port the farthest when it is near the center of the port vertically. As it passes center and the ring has to be pushed back into the groove, the most force is required in the beginning of that push which is at the edges of the port. As the rings moves closer to the bottom of the port, it isn&#39;t sticking out nearly as far, thus less pressure is required in that area.

I&#39;ll get some more photos this weekend of the piston and ring I posted earlier. Some shots from each quadrant may help illustrate where the wear occurs in relation to the cylinder.

It would be interesting to see the same from several other bad 800 rings for comparison.

It will be great if T.D. can have a ring or rings analyzed to see if the problem is primarily thermal or vibration?

SummitAK

I to am working on a 800 rotax (03 rev) and playing with Kelseys stuff, the timing issue has concerned me. I wonder how close the factory sets the timing , I know that you can advance or retard it and last year I had my dealer retard it 2 degrees but 2 degrees from what? I should find out this afternoon when I pick up my sled, I had them check the timing.

Doodoug
Process Coatings, Inc

PS: Swains new coating isn&#39;t really new, we have been doing that for 8 years+ with great results.

"As stock" my 800 was 1 1/2* advanced from spec. I am set right now at -2* per Kelsey. At first once the timing was reset, I thought it sounded much snappier when I loaded it up. But I don&#39;t here that anymore (mentally) and I can&#39;t say I noticed any seat of the pants diff. I haven&#39;t rode it much since then. One weekend is all I think.

As I understand this was a normall fix for the 670&#39;s too. ???


What is the width of the exh port? It looks to me (judging from the colorations on the piston) that prox 80% of the poor ring surface is NOT in the port area, but on the solid surface next to it.

I will pull my Y pipe off soon and see how these pistons look after a somewhat shorter season. (1000 smiles, mostly off trail) My compresion is still within a # or two from new. (Kelseys)

Doug, if the dealer doesn&#39;t know HAFAHITG he can get your timing screwed up. The pickup for the flywheel is fixed and cannot be adjusted so sometimes the timing is off a little bit. Doo checks them and stamps a number in the case. It&#39;s located on a flat spot on the backside above the recoil. This is a correction factor, the correction is made by reprogramming the mpem. Here&#39;s the correction numbers: 1= 0deg, 2= +3, 3= +2, 4=+1, 5= -1, 6= -2, 7= -3, 8= -4.

it&#39;s confusing but heres an example: my case is stamped 4, that means the motor tested 1 deg adv, so doo would have put an 5 into the mpem to correct it.

Ok now I did a mod and wanted to retard the timing 2 degrees, take it to the dealer his unknowing mechanic plugs his programmer in looks at the menu and see&#39;s -2 and punches up a 6. Now my motor is running -1.

Another example, a guy is trying to maximize his mods on the dyno, he has access to a programmer. He makes a pull at each timing setting to see what makes the best torque #&#39;s. Finds out that 7 works the best, so now he tells all his buds (whos motors had a different correction factor) that -3 is the way to! Not saying this is the case with rtk advise, kelsey knows his stuff.

The timing mark on the flywheel seem to accurate from what I&#39;ve see. Put a timing light on it at 3500, if it lines up your at stock timing.

We checked the timing the old fashion way with a degree wheel & dial indicator it seems that it was right on, then put in the correction factor to retard per Kelseys advice.


Thanks Doug

OK, here is an attempt at 4 photos showing a stock 800 ring from the exhaust, pto, mag and intake sides. Flaking is most prevalent at each side of the main exhaust port, though there is considerable flaking located completely within the rear pto intake transfer and near the ring ends (with a majority within the rear intake port).

Unfortunately the matching jug has been ported so port bevels are not available for comparison.

SummitAK

Mag side:

Intake/boost port side:

PTO side:

A quick way to verify if your timing is set correctly is to put a timing light on the case, there is a cutout to check.

Good job &#39;AK. It looks like a good case against bad port chamfering to me. Along with some excess heat. What do the rest of you think?

One of the first things I&#39;m going to do is mechanically check the timing with a degree wheel.

Well, I received SummitAK&#39;s flaked piston ring. At first glance it looks cooked. The deterioration appears to start at the bottom of the rail and progress upward. I measured the tension alongside a brand new ring and found it to be 8 grams less. I have also noticed that the molly filler is easy to pick out with a machinist&#39;s scribe indicating that the electro plate did not adhere very well to the ring. I have sent the ring to Canadian Vac Hyde Aero for analysis. Since they charge an outrageous amount to do rush lab work for aircraft manufactures, I elected to have the ring checked on a "when we get to it" schedule. I have also checked the the dimensional variance between a new ring and this one. Summit&#39;s ring seems to be torsionally twisted upward minimizing the path of heat the the piston ring land. I also found the ring end gap on two seasoned 800 cylinders to be .025". As soon as I get ring hardness and molybdenum filler quality verified by the lab, I&#39;ll re-post the results and any suggestions I&#39;m able to get from the pros.

I would like to add my findings to this topic as they deviate drastically from the above posts. I have a 02 mxz 800 TRAIL, Stock motor, 500 mains, non DPM with over 6000 miles, 150 lbs compression in each cylinder, original pistons and rings. Ring inspection: Ring sealing surface perfect with one tiny pitted area approximately the size of the ball in a ball point pen. Top of ring is just starting to flake the coating off. Bottom of the rings surface has changed to a brown/yellow but still intact. Cylinder inspection: great general condition. Hatching still present throughout cylinder. All ports are chamfered approximately 1mm. Piston inspection: These pistons DO NOT have any relief or cutouts at all around the piston pin areas. Just a hole large enough to insert the piston pin and a small access slot for piston pin clip removal.

If the cylinders are the same for the trail, X, ect... this could be a possible source for "non short circuiting pistons".
I hope this can help in some way.