85 psi compression on a new rebuild

[Jon B]

I have a 2002 Polaris Trail RMK 550. It had a crank seal go out. I had it professionally replaced from a great shop. I did the top end. I oiled everything really good, checked ring gap, put rings in with letter on top, followed torque specs, new gaskets and seals. I idle warmed it up and let it cool 4 times. I put a tank of gas through it mixed with oil and didnt go over 5000 rpm. Ran ok but a little sluggish. Rechecked head bolt torques and checked lower bolt torque by using starting fluid while it was running and didnt get any rev up. I am only getting 85 psi compression. Manual says I should be 120 psi. I tried two different compression testers. Any idea on what I could be doing wrong or what is wrong with this sled? Thanks for any help. This problem is killing me. I have been working on it a long time.

 

[slash]

The 550 is pretty straightforward, Do the pistons come to the top of the deck?
Are you using the OE style head and cylinder gaskets?
Base and head gasket thickness can alter comp.

 

[Jon B]

I thought it was as well. Yes the pistons come to the top and I am pretty sure they were OE gaskets. I dont think they were thicker at all.

 

[whoisthatguy]

Look for the O-ring on the compression tester to possibly be damaged. With the rings, the chrome faced ring goes on top and the darker ring goes on the bottom. One side of the ring is flat and the other side is beveled. The flat side goes down and the ring must spin easily on the grooves once in place, considering that the pin that marks where the ring joint lines up which tends to hang up the spinning. The inside of the cylinder should be honed to leave very tiny scratch marks that catch oil.