I have an issue with a project. I have some 6-32 UNC screws that I need to use. I won't get too far into specifics but this is standardized and I cannot use any other screw. The problem I am having is that I found a specialized nut that I need than only comes in metric. Unfortunately it only comes in M3 and M4, not M3.5 which is a close equivalent to 6-32. Although, after looking into it more, I think the thread pitch between 6-32 UNC and M3.5 is different anyhow. Since M3 is a little smaller, would it be possible to get M3 nuts and use a 6-32 UNC tap on it?

closed as off-topic by isherwood, Daniel Griscom, Tester101 Aug 28 '17 at 14:12

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  • IMHO Retapping a thread close to the one you have will be a mess. Try to find blank nuts to tap. Maybe the manufacturer can send you some untapped ones? – ArchonOSX Aug 15 '17 at 19:27
  • I've already messaged a few about tapping the thread I need but it will be a while until I hear back so the retap idea is a backup. But that's a good idea to send an untapped if they can't tap the correct one. I already mistakenly bought M3 nuts so I think I might find someone that has a tap just to see what happens. I don't want to buy my own tap if it's not what I need. – user2328273 Aug 15 '17 at 19:41
  • Are you kidding? Taps are cheap, especially the little ones. You're probably talking under $10 including the T-handle. – Harper Aug 15 '17 at 19:50
  • Yeah, I know but why spend $10 on something that might work if I can find out for certain first? If it works then I'll buy one. – user2328273 Aug 15 '17 at 20:23

First, check McMaster-Carr. If anyone will stock a specialty nut, it will be them. (you might also take a swing by Fastenal, but I find them odd to do business with.)

With a tapped hole, the diameter of the hole has a minor and major diameter due to the threads. The major diameter of a hole tapped M3 will be 2.980 millimeters or 0.117 inches, possibly a smidge smaller.

Once you cut a #6-32 thread into a hole, its minor diameter is .114" and its major diameter is .138".

Sounds like it will be fine, assuming your application isn't very high strength. You might want to run the nut through with a drill bit first, such as a #36, .1065", the one recommended for a 6-32 tap.

  • I think I may have already checked McMaster-Carr but since I can't recall I'll check again. Good point about trying the drill first. – user2328273 Aug 15 '17 at 20:21
  • Redrilling and tapping worked beautifully. Turns out I did have a 6-32 tap lying around already too. Thanks for the help. – user2328273 Aug 25 '17 at 19:12

The tap drill for a m3 is 2.5mm or .0984" the tap for a #6 is .1065" these are fairly close in size but the #6 is larger so there would be some thread depending on the strength needed there may be enough to do the project it would be worth a try since the fasteners are "custom". I would give taping a try to see if there will be enough thread to do the job I don't have any metric taps to measure the depth of the thread for a m3.

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