If the tread pitch/spacing doesn't match either a SAE or metric thread gauge, you could measure the thread pitch and thread angle using an optical comparator and the thread major and minor diameters using a caliper. Then, hopefully, you can determine what you have using thread charts. This is how I determined the flywheel set screw on my Doll 365/1 is 1/8 x 40 BSW. For measuring pitch, you can mark the threads with a pencil and then press them into a sheet of paper. Then you can measure using the comparator on the flat, transferred thread pattern.
Happy Steaming!
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