Hi competitive dad.....I measured my removed head volume with a perspex cover in what I considered was the standard way. They measure around 18-19cc.....miles away from 21.2cc!.....it turns out the 21.2cc is measured via the CIK method which TKM also use. So head installed on the engine, piston at TDC, with the TKM tool for screwing in the spark plug and filling with fluid from a grade-A burette. Yes not mentioned in the regs, Kelvin told me after I asked.
I can only go by what I have seen on race day, if you are below 21.2cc expect trouble.....if you are skimming the head it will only make the valve pockets shallower, so they will be quite safe, and pass the gauge checks, only criteria you need to be careful of when machining is the sparkplug protrusion tool, and the head depth tool, the latter is where the carbon build-up is a consideration.
Most heads are bang on the 74mm nominal. Machining off 0.2mm down to 73.8mm is well worth doing.....compression ratio is everything in these engines.
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