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Alberta Mike, None of the replys answers your question. I don't know answer either; but I'll make a guess. "Number" size drill bits is a reference to "Gage Number". In years gone by, many many suppliers [manufacturers] of materials (wire, plate, bar, rod, sheet, tools,...) created standard sizes [dimentions) - read THEIR standard sizes. Some gage sizes, even today, have retained the name of the that company (ie, Brown & Sharpe, Birmingham iron, Washburn & Moen, American Steel & Wire, U.S.Standard,...). The "Number" of number size drill bits refers to wire gage number. the actual size of the wire gage seems to follow the Stubs' Steel Wire standard. Probably, some drill bit manufacturer used steel wire based on the Stubs' standard to make drill bits. A Stubs' standard wire gage size 40 is .097" diameter...so the number 40 drill bit was .097" diameter. oooops, #40drill bit is .098" . oh well life wasn't perfect. I have no clue how letter size drill bits were named. #1 number size drill bit is the largest, so to go larger than #1 [smaller number] they probably just started using letters to name larger sizes. Notice letter drill bits stop at Z....if we want to put names on bits larger than Z, maybe we can use the names of the Planets, or star names, or baseball player names, or names of fish, or... PS; Ray, had the best advice about sizing holes for taping. His info is the straight stuff. If others tell you that you have to size (drill) the hole so all the threads are full depth, they are wrong. Using tools available, 100% thread depth is nearly impossible to fit. 60% is fine, 75% is tight, 80% calls for precission fitting, 40%-50% thread depth is plently good enough for non-engine farm stuff.
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