A common modification to some engines is to remove the head gasket and lap the head into the engine block. This is especially popular on small two-stroke engines such as small motorcycles as it can lead to a significant increase in compression and hence more power.

Basically, lapping the head involves removing the gasket and all its remnants, then applying grinding paste to one half or the other, and then rubbing the head against the barrel. This flattens the surfaces against each other, and grinds away the bumps. Finally, the surfaces are cleaned and a very thin layer of gasket goo or even just more grinding paste is sandwiched in the join. This forms a gas-tight seal.

By removing the gasket, the head is closer to the barrel, and thus the unswept area above the piston is made smaller. This increases compression ratio, and higher compression means higher combustion chamber temperatures, which means more power.

Until it blows up, that is.

I should explain this. Increasing the combustion chamber temperature and pressure puts more thermal stress on the head and valves. It also puts more mechanical stress on the seal between the head and the barrel... which you just removed. It's fairly obvious what's going to happen here if you go too far.