Thanks for your replies.

In this particular case I'm going to break in an O.S. Speed XZ engine. I think I read somewhere that this engine is set up for 30% nitro fuel. I'm going to use/race 16% nitro fuel.
The manual says that there are two head gaskets istalled (0,2 and 0,1mm). For low Nitro fuels the 0,2mm gasket should be removed. I guess they mean with low nitro fuels 20% nitro an less, right?

Doesn't the compression grow when removing gaskets? Since I've read that O.S. Engines are build very tight (Piston/Sleeve) isn't it better to leave all the gaskets in for the first few tanks an remove them later?