15/16 tap

johnny mack · Nov 18, 2017

I'm new to this site and new to machining. I ran into a problem that maybe some one would be so kind to help me out with. I have a 15/16 x 16 tap and I have no idea what drill size would be called for. I've looked on several chart but none of them have 15/16 listed ........thanks Johnny Mack

eKretz · Nov 18, 2017

A 7/8" drill will work. A quick easy shortcut when you don't know the drill size is to subtract the reciprocal of the threads per inch number - or the pitch - from the nominal O.D. So for a thread size of 15/16" - 16 you'd take 15/16" and subtract 1/16" or .062" and you get 7/8". If you want the "official" size with limits, get a Machinery's Handbook.

Larry Dickman · Nov 18, 2017

what ekretz said is usually close enough. FWIW, For a 3B thread, the actual minor is .870 / .878

robert123 · Nov 18, 2017

This website provides quick info. It even gives limits over thread wires for external threads.

UN imperial screw thread calculator

johnny mack · Nov 18, 2017

Thank you guys, what a quick response.That is a fairly easy calculation to make when your in need of a drill size. I'll bookmark
that other site for future reference..... Thanks Johnny Mack

EPAIII · Nov 18, 2017

As a check, my tap drill calculator gave me a value of 0.8766" for a 75% thread. That is only 0.0016" larger than the 7/8" size that the quick formula gave. Not bad for the quick formula.

It also told me that the 7/8" drill will give you an actual 77% thread. Of course, I doubt that a 7/8" drill will make a hole that is that close to the actual 7/8" size of your actual hole will probably give you the 75% thread or perhaps even less.

rcoope · Nov 19, 2017

As a general point, 15/16 is definitely under the heading of "weird sizes". McMaster, for example, carries taps in that size but not hex head screws. I'd be leery of using it unless you have to fit some existing screw in a single project of some historical nature where you can't imagine anyone in the future needing to replace the fastener. Otherwise you will drive some future person nuts. Or some existing person nuts in the future. Possibly yourself. The research lab machine shop manager where I cut my teeth as an instrument designer took 3-56 and 5-40 threads in instruments as a personal insult, so I was influenced by that.

johnny mack · Dec 1, 2017

The reason for asking is that i'm attempting to build a copy of a 1878 Sharps Borchardt and the receiver is taped 15/16x16 for the barrel. Thanks again for the help.

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