Question regarding Measuring and setting head clearance

SuperDave wrote:Lane:

On Cox engines it is usually done by either adding or subtracting thin copper head gaskets.

That's pretty much the case. We Cox users change the compression to account for fuel mixture, prop size, and wide variations in temperature. There are rules of thumb that work pretty well, such as adding a head shim(washer/gasket) for every 10 percent of nitromethane, starting with 1 shim for less than 20%(a poor formula for a Cox), 2 for 20%, 3 for 30%, etc. Cox parts were made with extremely reliable tolerances, so this may be the reason you're finding no information on the subject.

On the other hand, every engine has it's own performance characteristics, so experimentation on each one to find its happy compression spot isn't uncommon. There are lots of head and plug configurations that can be tried to tune an engine to its peak.

As far as decking the heads, the only reason I've ever done it it to ensure a good seal against the copper shims. Some people change the timing by shimming the cylinder height at the crankcase. There are various reasons for this, including eliminating sub piston induction so you can use a muffler, but for the most part it usually isn't helpful, and more than not it's hurtful to performance.

That's all I've got. Maybe someone with more engine knowledge will come along with a better answer. Paul Gibeault is probably our best expert at squeezing every last bit of power out of Coxes. If you PM him, he'll answer. Also see his Mouse Racer article pinned at the top of Cox Discussions.
Rusty