ORIGINAL: downunder

One way to check valve clearances at different piston heights is to simply push the valve down against the spring and see how much it moves. With an angled valve you'd have to work out the geometry to get an accurate figure though and a DTI would be rather helpful too . Or thinking about it, push the valve down by the spring cap and use feeler guages to measure the tappet clearance.

Using 20% nitro with a 13.5:1 compression is a recipe for disaster. Nitro doesn't tolerate much compression before it detonates and probably around 10:1 would be the limit for 20% nitro. Pure nitro is limited to 6:1. Theoretically methanol will tolerate up to 17:1 but you'd need a very cold plug to keep the ignition timing in check and then it might not like to idle. The ideal would be 17:1 and spark ignition .