1991 polaris 350L trailboss

[waynemags]

my brother in law has a 91 polaris 350 liquid cooled i think its a trailboss. anyways out yesterday riding and it quit. got it home and piston skirt is destroyed. looks like a complete rebuild. can cylinder be bored or is it nicasil.? any ideas would be great. thanx. cylinder doesnt look that bad but piston is destroyrd.

[DirtDemon]

I doubt that the Trail Boss has a Nikasil plated cylinder, so I would think it could be bored. I would at least hone it if there is no real damage and the bore isn't worn past the size of the piston. If it were me, I would probably just go up to the next piston size and put a fresh bore on the cylinder, just my 2 cents. You will want to get into the bottom end and make sure there aren't any pieces of that piston floating around in the engine. While you are in there check the crank and any bearings for wear. You will also want to look into possible causes for the breakdown. Check the plug and valves, the condition of these things can be a good indicator of any problems that may have occured.

[waynemags]

which valves are you talking about? we were thinking of an oversize anyways.

[DirtDemon]

The intake and exhuast valves, note the color of the valves and any deposits that may have accumulated on them. Check to see if they are seating properly.

[waynemags]

its a 2 stroke

[DirtDemon]

I should have known that. Especially after you asked "what valves?" I forget that the earlier Polaris quads were 2-strokes. In that case, the top of the cylinder and the combustion dome will give the same indication. It is likely that the cylinder was just a little worn and the piston skirt caught the edge of one of the ports and broke. I would still split the cases and inspect the crank and bearings in the bottom end.