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GrahamI'm sure you've covered this before, but my drawing suggests that the governor valve and the bore the the end of the cylinder are both 1" dia. Instinct suggests that the valve is slightly smaller, say 15/16", so that when it activates its sucked into the bore, but with a 30 thou annulus all round to choke the engine and slow it down. Otherwise, slightly larger, say 1 1/16" but with a few 1/8" air hole drilled in it.Even though I've done most of the engine, if you do get hold of a copy of your build notes, I'd appreciate a copy, if only to see where I did it wrong!Do these hot air engines run either way round? I presume they are not self starting, but need a good pull to get them going.