I have put in new pistons 3 times now and this thing is Goulding the side of the piston on the intake side every time. Stock, and bore over .040 2 times. Same thing every time. Wth

 

If the top ends have been done right,could be a frozen one way check valve located below the oil lines on the engine causing problems. Has a plug, spring and bullet valve.Cheap 5 buck manual can help and is shown under lower engine rebuilding,plus other checks you can do. 2004 Polaris Sportsman 500 Service Manual PDF Download

 

Top end is done to specs and the top end is getting plenty of oil flo

 

Then there has to be a reason why the pistons are doing this. Scoring of pistons on the intake side can be from lack of lubrication or too much clearance allowed on the cylinder bore and sometimes air leaks,similar to what happens on a 2 stroke piston.And these pistons have super tight clearances if you look at the manual. Top end may be getting oil,but bottom end may not be.That's' why I said to check the one way valve Several people have had problems with these. Look at the oil flow chart also.Plus did you check oil pressure before the first piston failed? Should be 20psi at 5000 rpms if the oil pump is ok.

 

Ok I'll check it out