i found this to check your head clearance if the first link doesnt work try the second and its the first vid. "Ultimate Nitro Engine Tuning DVD Sample " nitro_tuning_sample1[1]
http://www.radiocontrolzone.com/cars/video.asp

If I were you I wouldn't take out a shim using 30%, but i might do it using 20%. The engine is designed for 25%, you would probably get a lot of detonation if you took out a shim and ran 30%.

Yup- I wouldn't recommend taking a shim out. Detonation is the silent killer. But definitely try it- run it for half a gallon or so and check the piston and sleeve for detonation. If no sings your good to go. You geographic location, altitude, weather etc. has alot to do with the detonation.

Sky; when you say "detonation" what exactly do you mean ? is it pre or post detonation you are referring to ? or something else ?

pre detonation. The motor fires earlier than it should.

If you see pitting on top of your piston or head button then that is a sure sign of detonation.