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Discussion Starter · #1 ·
This snowmobile was given to me along with an 88 400. The 88 is running good now. The 86 is all together after a top end rebuild and it has no compression on either cylinder. The engine has not fired once yet. What would cause no compression in both cylinders? It has maybe 5psi in either side. Would the crank seals be so bad that it would have this little compression?

When it was given to me, I was told that a screw had been taken out of the oil pump of the 86 to put in the running 1988 400 but I cannot see what screw is missing on the 86. Would this cause the loss in compression? Thanks!
 

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mikey007 said:
This snowmobile was given to me along with an 88 400. The 88 is running good now. The 86 is all together after a top end rebuild and it has no compression on either cylinder. The engine has not fired once yet. What would cause no compression in both cylinders? It has maybe 5psi in either side. Would the crank seals be so bad that it would have this little compression?

When it was given to me, I was told that a screw had been taken out of the oil pump of the 86 to put in the running 1988 400 but I cannot see what screw is missing on the 86. Would this cause the loss in compression? Thanks!
Hmmm no rings on the pistons would cause a NO compression reading...are you testing the compression with a known good gauge?
 

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Discussion Starter · #3 ·
RAMROD said:
mikey007 said:
This snowmobile was given to me along with an 88 400. The 88 is running good now. The 86 is all together after a top end rebuild and it has no compression on either cylinder. The engine has not fired once yet. What would cause no compression in both cylinders? It has maybe 5psi in either side. Would the crank seals be so bad that it would have this little compression?

When it was given to me, I was told that a screw had been taken out of the oil pump of the 86 to put in the running 1988 400 but I cannot see what screw is missing on the 86. Would this cause the loss in compression? Thanks!
Hmmm no rings on the pistons would cause a NO compression reading...are you testing the compression with a known good gauge?
I just re-did the test and and both sides have a little compression, between 15-30psi a side. I tested my other 400 and it was 110 a side. It could be the rings, I haven't looked at those yet.
 

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Could it be that rings haven't seated? Do you have the right size pistons; As in maybe the cylinders were bored but you're still running standard bore pistons?
The condition of the crank seals will have no effect on the compression reading, Compression readings only take into account the combustion chamber area.
 

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Discussion Starter · #5 ·
Polaris_Parts_MN said:
Could it be that rings haven't seated? Do you have the right size pistons; As in maybe the cylinders were bored but you're still running standard bore pistons?
The condition of the crank seals will have no effect on the compression reading, Compression readings only take into account the combustion chamber area.
How do you seat the rings other than putting oil down the cylinders if it won't start? The pistons have .010 stamped on the tops of them so I do think they are the right pistons. I took the y pipe off to look at the rings and they look okay from what I can see. I am at a loss with this thing!
 

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mikey007 said:
Also, is it possible to put the rings in upside down? I can't think of anything else.
Yes, there is a beveled edge on the inside arc of the ring that should be facing up.
 

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Discussion Starter · #8 ·
Polaris_Parts_MN said:
mikey007 said:
Also, is it possible to put the rings in upside down? I can't think of anything else.
Yes, there is a beveled edge on the inside arc of the ring that should be facing up.
I took the rings off but there are absolutely no markings whatsoever to determine the top from the bottom. Do you just have to eyeball the beveled edge?
 

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Discussion Starter · #9 ·
I am at a loss. The rings are good, the pistons are good. It must be the cylinders. I can pour gas on top of the cylinder and it disappears in seconds. Maybe they weren't honed or bored properly.
 

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Discussion Starter · #10 ·
Okay, I have some pictures for you guys. Let me know what it looks like to you. I can squeeze the ring tight to the cylinder and the rings only go on one way.

http://www.flickr.com/photos/[email protected]/
 

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well its overlapping haha. it should be 13 thousands but correct me if im wrong. you need a feeler guage
 

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check the ring gap every 1-4 inch down the cyclner see if it changes the cyc usaly weres more down the cyc if the gap changes to much u will to over size ur cyc
 

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Discussion Starter · #18 ·
polaris-in-labrador said:
check the ring gap every 1-4 inch down the cyclner see if it changes the cyc usaly weres more down the cyc if the gap changes to much u will to over size ur cyc
I will have to check that. The pistons are already .010 over.
 

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Discussion Starter · #19 ·
I fit the rings down the cylinder walls today and the ring gap does not change at all from what it is in the pictures.

Is it possible that if the timing was off, it wouldn't fire? Or it definitely should have more compression regardless?

I am going to take the rings and cylinders back to Harley where it was rebuilt and see what they say.
 
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