Originally Posted by
Grinder
Feel the bearings with your finger. If they are smooth, then I would recommend a new piston/sleeve/rod/button. Make sure you have at least .50mm of head shims. That makes a head clearance of .50mm. I run .55mm of shims in my picco based engines. .12, and .21 engines.
The picco .21 engines including the idm come with .40mm of shims. That's too low for 25% nitro. It's too low for 20%. The stock clearance is ok for 16%.
thanks for your input, tryed using a small amount of metal polish on the top of the piston, made it abit smoother but there is still those deep pits in it, really at this point it needs a new Rod/liner/piston/head button, the bearings are not the smoothest, but seem to be ok