2002 Polaris rmk 700 low compression after rebuild

[Noah Vanscharrel]

I did a full rebuild on my 02 rmk 700 after the piston skirt went out after rebuilding it I ran it for two start ups and then it's starting killing itself then I did a compression test after it wouldn't start and both cylinders were at 60 psi I tore it apart to find the skirts on the new piston were scratched a bit but there's nothing to catch a finger nail on the pistons but I checked the clearances and they were .004 and .0045 do u think it's the rings or are the cylinders bad it has just over 5000mi any info will help thanks I'll include pics of pistons and cylinders.

 

[davesilb]

That is a lot of scuffing on cylinder walls! Did you do a hone job on cylinders before installing new pistons? If not you probably have ruined the new rings. Most people would get new plated cylinders when having that much scuffing. However if you are not looking for top performance then a ridget hone job with new rings might get you going again. Be sure to run some extra oil in fuel to begin with for the first tank or so and make sure your oil pump is supplying approximately the correct amount of oil by watching how much it is using from the oil tank reservoir .Did you make sure there was no loose pieces of piston skirt still in crank case? What about carbon peices from old pistons and head?

 

[Noah Vanscharrel]

That is a lot of scuffing on cylinder walls! Did you do a hone job on cylinders before installing new pistons? If not you probably have ruined the new rings. Most people would get new plated cylinders when having that much scuffing. However if you are not looking for top performance then a ridget hone job with new rings might get you going again. Be sure to run some extra oil in fuel to begin with for the first tank or so and make sure your oil pump is supplying approximately the correct amount of oil by watching how much it is using from the oil tank reservoir .Did you make sure there was no loose pieces of piston skirt still in crank case? What about carbon peices from old pistons and head?

I did hone the cylinders before I rebuilt it and cleaned out the crankcase

 

[Noah Vanscharrel]

I did hone the cylinders before I rebuilt it and cleaned out the crankcase

And what is a ridget hone?

 

[whoisthatguy]

I did a full rebuild on my 02 rmk 700 after the piston skirt went out after rebuilding it I ran it for two start ups and then it's starting killing itself then I did a compression test after it wouldn't start and both cylinders were at 60 psi I tore it apart to find the skirts on the new piston were scratched a bit but there's nothing to catch a finger nail on the pistons but I checked the clearances and they were .004 and .0045 do u think it's the rings or are the cylinders bad it has just over 5000mi any info will help thanks I'll include pics of pistons and cylinders.

Many people have undamaged cylinders rebuilt with nicosil coating at no more than 3000 miles. Those look pretty ragged. But you can clean up a cylinder with the "3M Scotch Brite 4" Paint and Rust Stripper Disk. If you put some 2000 grit sandpaper and spin that around the inside of the cylinder with that stripper disk, you can give it a mirror finish, providing there are no grooves. You can also buy two new 800 RMK pistons and rings as a kit on Ebay for $110 from sledpartsguy. So if you get new pistons, rings and smooth down the interior cylinder walls and get 130 psi compression after smearing oil on the cylinder walls, I would call that good. To do it right, you nicosil the cylinders and use that stripper disk to clean up the piston walls so they are perfectly smooth or buy 2 new ones and keep those as spares. Always smear oil on the cylinder walls before reassembly, and get some in the piston rod bearings, and cylinder bearings.

 

[BeartoothBaron]

It looks pretty rough to me, and I wonder if either it was too worn to begin with, or if you may have honed away too much of the plating. I've heard of good results from just a light hone, and with worse top end failures even using muriatic acid to remove aluminum deposits followed by a hone, but at some point the plating is just too compromised. If it were me, I'd have them re-plated and give it another go with new pistons. The only thing I can think of that'd cause a rapid failure like that is if it were assembled dry (like whoisthatguy said, you want a coat of oil on everything that moves or sees movement) or wasn't getting oil. If you can't find proof of that, and maybe even if you do, I'd lean towards having the cylinders plated and honed before you go any farther.

 

[Noah Vanscharrel]

How easy is it to get them plated maybe would I be better off with just ordering some new cylinders?

 

[whoisthatguy]

How easy is it to get them plated maybe would I be better off with just ordering some new cylinders?

There are cylinder rebuilders in every major city. You can also buy already nicosil plated cylinders for about $250 each and give them your old cylinders as the $100 core deposit. You won't be able to afford new cylinders from Polaris because they hose all purchasers of their snowmobile parts. If you are putting $500 into replated cylinders then you better start over with new pistons and rings and keep your barely used ones as spares.