never mess with shim unless u changing nitro content to match, when u change shim u change the detonation burn on the top of the piston, the burn explosion in our motors burns from the center of the piston outwards if u change the shims and leave the same nitro then it no longer will burn from center out and that's when people get the top of there piston looks like its deteriorating, it eats away at the top of the piston