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What I do is to eyeball the drill bit next to the tap. The drill bit should be just inside the threads.<br /><br />Tap /\/\/\/\/\------ Drill bit<br /><br />Tap \/\/\/\/\/------ Drill bit
Are you sure an 8-48? I've never seen that thread in UNC/UNF/UNS. 8-32 is a #29 bit, or approximately 9/64. <br />I have a chart from the Starrett catalog right next to my drill press. It's a lot like this one:<br /> http://www.iroquois.free-online.co.uk/drilltap.htm
Hi Paul, yes the tap is 8-48, it's an old Winchester gun sight. A real odd ball size. Still hoping someone can help. If not, I was thinking of using a #29 on a test piece. Thanks again.
I see what you mean. There's a bunch of tap drill calculators online that you can put in whatever parameters you want. When I plugged in 0.1640" diameter (#8) and 0.02083 thread pitch (48tpi) they say for National threads the hole should be 0.144 and for Unified threads 0.147. That works out to a #27 or #26 bit, which fits with my instinct; bigger holes for finer threads.<br /><br />A test would be a very good idea. I couldn't find a table with an 8-48 tap so I'm not 100% sure of the thread type. Chances would seem to be it's a National thread though, since I'll bet your gun is older than the unified system.
Hi Paul, thanks for the extra reseach. The gun is from 1929-1931 era so I don't where that falls in relation to the unified system. I may just have to try tapping a hole with #26, #27 and #29 to see which looks best.
