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I just bought a 1996 Polaris Indy 500 SKS and after a compression test I realised it needed a new top end. So I removed the head and found a nice chunk out of one of the pistons. (After doing an through inspection of the lower end for piston parts) I measured the cylinders to see what size I need to bore to and what size pistons to get, I found that it was already bored .5MM over the original 72MM bore. So I ordered 2 new 73MM "Parts Unlimited" pistons kits and top end seals from Dennis Kirk and I had the cylinders machined to 73MM.

I know I need to have the head domes machined to match the 73MM bore.. But When the head was removed I noticed alot of damage from the piston chunk on the PTO side cylinder head dome. So I ordered a used head from a snnomobile salvage yard. When I recieved the new head it appeared that it has been milled down on the mating surface and the domes are now alot closer to the pistons that they were. I have no idea how much was removed. I had thoughts of shaving my own head since its all appart, so this is not a major problem for me.

Since the head is shaved I need to now worry about having proper squash. I know to have the head machined to match the new 73MM bore, but I do not know how deep to have him machine the squash ring?.. I am thinking of having the machine shop cut the dome to 73MM and only cut it only enought for me to take a squash measurment and then give him a final number to make my squash perfect. This is where I get a little confused, how do I know if the head has been shaved to much for me to use?. As long as I can get the squash right then it is ok?, what if I have to go very deep to get the correct squash?I do not want to adjust timing and I do not want to do any other mods.

I do not know what is stock squash?
What is safe squash?
What if I run 93 octaine?

When puting the head on to check squash, is it ok to use the old gasket so I dont ruin the new one?.. do I need the engine at opperating temp to check squash?

I measured my stock heads domes and I got 71.8MM, is it like that for a reason?. should I have my new head machined to 72.8 or just 73MM?

Another odd question I have is since my cylinders are now bigger, do I need to get a diffrent head gasket with bigger cylinder holes or will the stock style one still work?

I am sorry for all the question, thank in advance for taking the time to answer them.
 

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Squash is a gourd, allot like a pumpkin. You are dealing with a snowmobile not vegetables.
I also think shaving your head because your sled is torn apart is a bit extreme but hey, each to there own.

You’re looking for "Squish" height.
You also are changing your compression ratio by dealing with "shaved heads".
The cc- displacement has changed and you’re essentially creating a new motor size.

Your cylinder bore is on its second machining operation etc. and you have found some shaved heads at a bone yard.
You’re dealing with a pile of hodge-podge piece together parts and everything that goes along with that.

1- why did the sled burn down again?
2- Can you be 100% sure your crank etc is a-ok?
3- Were those cylinder originally nic coated?

Without getting into a crap load of what you’re dealing with and the scenarios such as porting, cylinder volume, timing etc your facing, I can offer a real world bit of advice.

Your best bet is to purchase a set of stock jugs, piston, rings etc.
Use the original heads if they were not machined to match the first bore oversize and are not cracked or terribly pitted.
If the crank is straight, balance in phase etc. put a new set of bearings in it.

Your other option is to replace the entire motor with a good used one-

This will eliminate your need to modify everything else because that’s what you’re diving into.
I am not trying to discourage you and there are some pretty handy engine mod guys on this site that may help you out. It’s just that you really have to look at where you’re at with this engine.
 

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Discussion Starter #3
I do not really want to use the shaved head because I do not want to mess with timing and race fuel and the headache your talking about. I will see again if I can return it but I dont think they will let me. I do not want to use my stock head because it has alot of damage.

The piston lost a chunk rigth bye the exhaust port due to a lean condition. I have the carbs off and gutted and soaking in carb cleaner. I purchased all new jets that polaris recomend for my elivation and temp.

I can not be 100% sure that the crank is ok as I did not take it appart, but I am about 90% sure that there is no chunks (that I can see) in the crank case. But with a sled this old I am ready to take a chance at it.

I have no idea if the pistons were nic coated. I dont have the original pistons, is there a way to find out? What does this mean?

I have already purchased the pistons and the cylinders have already been bored so geting a used engine is out of the question for me at this point. Also since I have all this done already I am going to use it. If it were a newer sled I would definatly go that route but for now I am going to just use what I have.

Even though my head is shaved, would it be posable to cut the squish to match factory and compression will be the same?..

Thanks for the help and input.
 

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I just happen to have a 488 out of a '97 Indy 500 sitting in my garage. I took it out of my cousin's wrecked sled a couple seasons ago, sold off the recoil, cdi, water and oil pumps. Other than that, it is just sitting there waiting for me to tear it down and do something with the rest of it. This motor had 5300+- miles on it when the sled was wrecked.
Don't know where you are located, but if you need parts off it, let me know what you want and a deal can be arranged for little more than shipping costs.
 

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Discussion Starter #6
Yea, I have been saying the wrong word the whole time. LOL..

Anyway, Did polaris use the same squish for all theyre engines?.



I called the yard that I got my head from and they put me in contact with the owner of the sled that the head came off of. He is an employee theyre. He said it is bone stock and it has not beed shaved at all. He said that its a posability that diffrent years had diffrent compression. Could that be true?.. I took some measurments from the gasket area to the dome right off the squish ring and my stock depth is 1MM, the depth on the new head is .80MM (Wich means its the equivelant of 10/1000's of an inch shaved over my stock head)

Even if someone was to use the recomended procedure to remove material to make it true with sand paper on glass, you could end up taking off 10 thousants. I dont think this will really effect anything. Do you agree?

Also, I asked the guy about extending the domes to 73MM to match the bore, and he said that when working with 1MM diffrence, it should not make a diffrence, he said that once you start using 2MM overbore then you need to start worrying about it?. Is this true also?
 

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.010 on the squish is a ton. you want to be within .002. taking of .010 to true your head up is unlikely. it would be more like .002 to .004. if it was mine id mess with it, but im looking for each 1/10th of a horsepower i can get.
 

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Discussion Starter #8
oh. Well did polaris use diffrent heads with higher compression on diffrent 488 machines or is the perosn I bought it from just trying to keep a sale?

I guess what I will do is put the head on with the old gasket and measure squish, then I will do the math and see how much I need the ring cut to make it perfect. then I will bring the head in and have it macined.
 

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1.00mm = .0394"
Millimeters divided by 25.4 will give you your inch conversion-
Inches multiplied by 25.4 will give you the millimeter-

I think your getting screwed up by your conversions.
A millimeter is huge when talking machining surfaces or oversize bores.

You will more than likely need to adjust your timing because your squish height is going to change.
Squish affects the cylinder volume, which affects the cylinder turbulence, which affects the fuel ignition because the compression ratio has changed.
Even if you compensate by adding or removing base gasket thickness you will still need to adjust the timing because you are moving the ports.
In turn your jetting and clutching will be out of wack and your pipe may throw you for a loop.
Look at it this way, when you’re done you may be an expert in reading plugs, piston wash, clutching and carb jetting.
 

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wow taxicat, you must be afraid to even add gas. it will add weight, which effects your suspension travel, so youll need heavier springs and it will also raise your cg, so your sled will roll more in the corners, not to mention the enviromental impact of your sled burning more fuel because it weighs more. :) mike
 

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Naw, Gas is ok. It's the soda, cheese burgers and chips. They will go straight to your butt which will increase the diameter of your clothing causing the suspension to collapse and bottom out.
This will then cause you to stop and have more soda, cheese burgers and chips.
It's one very ugly cycle.
I won't even mention what beer does.
 

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Discussion Starter #13
I have been running the head with no problems with out ever measuring anything. I have 82 miles on the new top end and its runing amazing.
 

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I was going to say the same thing-

Check your wash and plugs-

Running "amazing" may be true however, I would bet it's not running how intended.
I could put different heads etc. on my sled and make it run good but not right-
Unless you can compare the sled to how it used to run, you couldn’t honestly say its spot on.

You may be able to go from amazing to outstandingly incredible-

It sounds though like you'r on the right track.
 

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Discussion Starter #16
I have not looked at the pistons yet. I have a tool for looking in small places in my tool box at work. I will grab it monday and do then.

I would measure squish but I have no idea what it should be, I have posted on 3 diffrent forums asking, and called 2 diffrent dealers.. no one has an asnwer for me.

I have been checking the plugs alot and they tell me im runing a bit rich but not bad. I have all stock settings and jets in the carbs wich is uaually rich for safety. once I get to 100miles I am going to re jet it.
 
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