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Discussion Starter #1
Last winter the mag piston on my mxz 500 siezed due to the check valve jamming on that oil line. I put in a pair of wiseco pistons,the cleareance measured 4.5 thousands. Now when I looked through the spark plug hole I noticed two marks on both cylinders on the intake side. I pulled the cylinders and both pistons have scuff marks along the entire piston on the intake side only. They look very much like four corner cold siezure marks, but only on the intake side, the exhaust side is perfect. Also the scuff marks are right under the ring end gaps. Would someone know why this would happen in just under 300 miles. I want to rebuild it again, but don't want this to happen again . Thanks for helping with any advice. I also give it adeqaute warm ups, and don't drive that hard.
 

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not sure if this is of any use or relevance but there were some piston scuffing problems on some 99 MX6s. this was apparently caused by ingesting snow which washed away the oil on the cylinders. SD has since remedied the problem with moly-coated pistons (and more screening for the air intakes). You may want to consider having any new pistons moly-coated in order to avoid this situation. Hope this helps.
 

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I had my mxz 670 seize a couple years ago and couldn't figure out why. Then for the heck of it I decided to check my jets they are staggered. Sure enough I had screwed up cleaning the carbs and swithed them. If your 500 is a 99 then I believe it has staggered jets. On a 99 mxz500 I believe it has a 300 main jet on the pto side and a 280 on the mag side. I would check that first, if you haven't already. Your intake and carbs should have corresponding dots on them (blue & red). Also make sure the right jet is in the right carb.
 

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Snow indestation would doo that, maybe a poor air box seal, but whether or not thats the real problem I wouldnt stake my life on it.
 

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Discussion Starter #5
Since it siezed the first time after I corrected the oil supply problem, I also jetted up to 320 pto and 300 mag just in case it wasn't only oil related. The sieze marks now are directly under the ring end gaps. The gap is 16 thousands. Could combustion gas be getting squeezed out these gaps and somehow melting the pistons here? Or could a too rich condition be washing the oil off the pistons and sticking them here, just like someone mentioned snow injestion doing?
 

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Below the ring, lack of lube, dirt or cold seizing, above the ring stress is detonation and meltdown wish I could be more helpful but a wise person once said "there are no easy answers".
 

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In my opinion, its lubrication failure due to wash-out AND clearance.

The reason that I am basing my hypothesis on this is because of the tell-tale signs of wear and setup.

First off, your combustion is slightly colder than it can be, so its not a heat related issue (you jetted UP 2 sizes from factory safe jetting)

The only markes are on the intake side, This takes the blunt of the force during your power stroke.

your clearances are slightly tight. .0045" is a minimum clearance for pistons. .006 is a safer clearance.


So adding all these issues, we can elliminate Combustion and heat issues.
We can also elliminate improper warm up and 4 corner scuffing,
We also know its on the hardest working side of the piston.

So that leaves us with lubrication.

There are several reasons why you may have had this failure.
- You may have used cheaper oil in a jam (ran out of the good stuff at the gas station). Cheaper oils loose lubricating properties under high stress loads.
- You may have washed off the oil film by ingesting snow.
- Or your clearances are too tight to be able to develop a good oil film.

But remember, these are only my theories and my 2 cents. I could be wrong. Unless I got the parts in out failure analysis lab I would not sign off on this being the only solution to your problem.
To quote Ballsout1 " I wouldn't stake my life on it"


Post a picture, it would really help.

Good luck!
 

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Discussion Starter #8
You may have the correct answer or answers. The oil I was using was regular bombardier oil but I mixed it a little once or twice with some poaris regular oil, because I had 2 sealed jugs of it and wanted to use it up. Also you mentioned a too tight of a clearence. These are wiseco pistons and when you mentioned clearences I remembered I read somewhere that wiseco needed more clearence than cast.
 

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What happens with Wiseco, is that they are made slightly bigger than stock. They are normally used on engine that need a rebore. You also need to specify to the person doing the bore job that you are running wiseco, that way they open it up a little more.

I have seen racer after racer blow engins up due to wiseco pistons and too tight piston clearances. usually very prone to 4 corner type failure because of improper warm up.
 

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Well man who did you have rebuilt your engine or did you. My guess is that someone pushed the piston ring pins into the piston and the ring endgap is not riding on the sides of it but rather the pin is pushing the piston ring out to tight against the cylinder. Causing scuffung. This is my guess since only one cylinder is doing this.
 
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