Still getting low compression (20psi) on 2002 scrambler 50 after new cylinder, piston and rings and head with new gaskets. Have not done a crankcase leak down test. Will a failed leak down test cause low compression????

 

Did you buy genuine Polaris parts? I suspect it will be a Minarelli engine made by Aeon, so Taiwanese anyway, but aftermarket cylinders etc, can be badly machined and give high or low compression. It is worth checking if the crankshaft seals are leaking. This would drop compression but fuel/oil/air leaking out would be streaking down the cases anyway. If they are OK, you may have got something wrong when you assembled it, a ring not against it's peg will break as you slide the piston on. Must be 15 years since I did one (Polaris 90) so I can't remember if the rings have a top side, but fit them upside down on a Suzuki LT80 and you will wreck them.

 

Thanks for the reply Merryman. No, not genuine Polaris parts. I have assembled the engine several times using two new pistons and rings with both the new and original old Polaris cylinders. The old original polaris cylinder has no wear marks on it at all.I get the identical compression reading each time (about 20 psi) no matter which cylinder or piston arrangement i use..The rings will only fit one way and they do fit against the pegs as they should and are not upside down. Also, the "arrow" on tne piston is pointed toward the exhaust side of the cylinder as it should be. Oh yeah, installed a new head and reed valves and new carb also and of course new gaskets and applied the correct torque settings when assembling. I do think that what I have done thus far is correct. Guess the next thing is to check the crankshaft seals. I guess its on to check the leak down test. Likely will never again use a "leak down test kit", so, will try to build one instead of buying one. Thanks again.