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There are machine-screw standards that match other sprinkler stuff. Residential brass (1/2-inch) impact-head nozzles came in two nozzle thread patterns, and the coarser of the two thread patterns matched a 3/8 x 24 UNF screw. The finer-thread nozzles were a match for 1/8 x 27 pipe threads, and maybe that's where 27 tpi stuff came from.
Your 'standard' Richdel/Irritrol solenoid has 3/4 x 20 threads, and that matches an extra-fine thread (UNEF) standard.
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