The normal,or usable range on for that motor is 119-185 psi.Warm up the motor ,install the gauge and with the throttle wide open Kick the motor over until the gauge stops rising,the compression is the highest obtainable(straight from the shop manual) then release,if you are in the upper area of the range then I think you have a winner on your hands.Record the reading and take it every so often,when it drops by 15-20% its usually time to check the top end.Most of the time new rings are the norm if everything measures ok.If you are running nice and clean and compression is still good ,take the top end apart once a year to clean the kips valves and replace the rings(might as well since your in there) and do measurements,if the valves are allowed to gum up they can strip the teeth on the 2 side valves.Stick with the stock piston as long as the clearances are still within spec,I have heard that some riders are getting as much as 10000 miles out of a stock piston with these bikes,not to bad for a 2-stroke.Not sure what the heck went wrong with the 220. :(