I have an 04 600 ho summit. Per recommendation from a former ski doo mechanic I installed a thinner base gasket for the cylinders (i believe the 5 hole gasket). If you have the cylinders off replace the rings. He also recommended gutting the air box and going with 20 pilot jets, for high elevation riding I had to move the carb needles up one clip to lean out the motor a little, the 20 pilots made it run fat. I also have v force reeds, hps can, and dynamoe joe clutch kit. Sled really rips for a 600, I was running with a guy who had a big bore 800 xp and he kept commenting on how strong my 600 was. Pretty inexpensive mods for the 600, but going to an 800 rev is still very economical right now. 600 is a more reliable motor though...