4-36 were used a lot in pick up roller of passenger cars and otherwise infrequently. 4-40 in O gauge and 6-32 in standard gauge. I have taps for all of these sizes so can freshen the threads. The best source for screws I have found and the most cost efficient is Mcmasters Carr. Carry all sizes including fillister head 4-40 used in prewar O.
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