ORIGINAL: yakfish
Most of our car engine come with shims installed between the piston sleeve and the head button. you can then add or remove a shim to either raise or lower the compression ratio without having to do any machining work.
You have to understand a few things when dealing with head shims. First off, just taking a shim out (say you have 1 .1mm shim installed for 20% nitro) will change your head clearance. Head clearance is the space between the bottom of the head button/head (the bottom of the squish band) and the top of the piston. If you pull the shim out and the head clearance is too small, the piston can hit the head/head button. One must measure the head, cylinder with piston at TDC, and factor in the shim to determine head clearance.
Take a Traxxas 3.3 for example. The stock shim is .011", and the head clearance with shim installed is .014". Take out the shim and you get .003". This is the bare minimum head clearance you should have. Now, with the shim removed, the compression is raised, but not a lot. I figured one of my 3.3 engines to have 9.63:1 compression (effective, not geometric) with the shim installed and 12.1:1 compression with the shim removed. Some engines must have a shim installed to maintain the proper head clearance, with some cases having less than .003" clearance. You can just pull the shim out and run it but without knowing how much room between the piston and head you have, you could wreck the engine. Get a Vernier caliper if you dont have one. They're a valuable tool when dealing with model engines. Determining compression ratio is easier with a small 1/2cc syringe.
I do not bother taking Geometric compression into account, only trapped compression or effective compression ratio.
Nitromethane and Methanol are hazardous. I buy Methanol locally, and I ship in the oil and Nitro. Some places dont charge the hazmat fees for nitro and some do. Shipping in quart bottles usually thwarts the hazmat fees. Look up Torco Fuels on Ebay. The last time I looked, a gallon jug of 100% nitro was $42 with free shipping. Yes, I said free shipping. I got my nitro in a 80/20 mix of 80% nitro 20% methanol for $13.50 a quart from FHS Oil Supply company. I will use the Torco stuff next time I order nitro.
Downunder: I stand corrected on machining the top of the sleeve. I figured taking a small amount off the head and sleeve would leave a little more meat on the head for sealing and thermal transfer. Isnt the ideal compression ratio for zero nitro fuel somewhere around 13.5:1 to achieve similar performance to using 20% nitro?