Charles,


Don't use one!

Adding a head shim not only reduces the compression ratio, it also increases the squish clearance, essentially changing the design of the engine.

The squish allows the engine to run the high compression that it has, without detonating and with high power.

It is a much easier task to just get any commercial 0-5% nitro, Castor-synthetic mix fuel (I use and recommend 20% total oil) fuel. If your LHS doesn't carry it, go to another LHS, or have yours order it in, so you will not have to pay Haz-Mat charges.


The extra-head-shim approach is only justifiable if you are on a desert island...