The headclearance is setting the compression. These Nitro engines are working on self combustion and as with any other engine the ignition timing is very important for maximum (and reliable) performance.

Ignition timing is set by:
- compression, a higher compression gives an earlier ignition
- plug, a hotter plug will give an earlier ignition
- nitro content, more nitro will give an earlier ignition
- outside temperature, hotter will give an earlier ignition.

Nitro content must be corrected with the headshims, the outside temperature must be corrected with the plug.

A standard .12 in Europe is set for the use of 16% nitro, running on 25% needs 0.1 or 0.2mm extra headshim. 10 degrees celcius higher temp outside needs 1 degree colder glowplug.

These are just standard indications to get the right ignition timings but you can always play with it, an early ignition will give more bottom power and and later ignition a tiny bit more rpm.