The other day while doing further work on my son's Subaru, I drilled a hole in a piece of angle iron then started using my 1/4-20 tap to cut some threads in the hole. The idea was to have a threaded hole which I can screw a bolt into to secure the upper portion of the vehicle's new radiator. During the process, I broke my tap. That really sucks and I wondered why I might have done it. I started thinking I may have not drilled the correct size hole prior to starting to run the tap. It may have been too small and therefore might have caused the breakage.

My question is: What is the proper sized hole to drill prior to using a specific tap? I was using a 1/4-20 tap, but I'm sure it may be useful to have a list from a 1/4" up to 1" tap size. I wouldn't think it would matter between coarse or fine thread. Also, if someone were really industrious, they could align it with the metric equivalent sized taps as well.

  • 4
    Even in a correctly sized hole in aluminum (drilled on a Bridgeport milling machine), I've broken taps. Using sufficient lubrication and making sure to take small "bites" is essential to ensure you don't break taps. Small "bites" meaning turning the tap a turn or two, then backing it out and then continuing to tap, repeat until the entire hole is tapped. – Shamtam Jan 13 '17 at 16:56
  • @Shamtam - Great tapping points. Exactly as I do it. – Pᴀᴜʟsᴛᴇʀ2 Jan 13 '17 at 16:59
  • 2
    It absolutely matters between fine and coarse threads, as their minor diameters are different for the same given major diameter. For reliability, use a nice thick cutting oil, and any time it's possible use a spiral point HSS plug tap. In production tapping we never use hand taps, because they trap chips and break. – Lathejockey81 Jan 14 '17 at 3:20
  • @Lathejockey81 - Thanks for the input! I'll definitely keep that in mind. – Pᴀᴜʟsᴛᴇʀ2 Jan 14 '17 at 3:21
up vote 13 down vote accepted

https://engineering.purdue.edu/ME463/Course%20Documents/Design%20Resources/starrett-inch-metric-tap-drill.pdf

This is my favorite chart for tap drill sizes. The chart incorporates tap sizes and the drill necessary for drilling before tapping threads into the hole. There is also a metric section off to the side.

1/4 20 would be a #7 drill bit or 0.2010 inches.

Here is a smaller version as a picture

enter image description here

  • I added a better quality picture ... I hope you don't mind. – Pᴀᴜʟsᴛᴇʀ2 Jan 13 '17 at 15:19
  • BTW - This was exactly the info I was looking for. Thank you. Now I just need to get a replacement tap, lol. – Pᴀᴜʟsᴛᴇʀ2 Jan 13 '17 at 15:20
  • Any good ol' boy'll just hold drills up next to the tap 'til one looks 'bout right... – Bob Jarvis Jan 13 '17 at 16:56
  • @Pᴀᴜʟsᴛᴇʀ2 I don't mind at all, glad this helped. – vini_i Jan 13 '17 at 18:52
  • depending on the application (and this one sounds not critical) you can go up a size, which helps in this case since its a lot easier to find a 13/64. Curiously what size did you drill to begin with. – agentp Jan 13 '17 at 20:12

There is a formula for it.... take the nominal size (.250") and the reciprocal of the threads per inch (20TPI = .05). .250 - .05 = .205, the drill size. Here's another 3/8-16.... .375 - .0625 = .3125 . Metric is even simpler you don't use the reciprocal at all... M8x1 is 8-1 = 7mm M5x.8... 5-.8 = 4.2mm.

Of course, many of the SAE sizes require odd-ball drills and you have to look them up in a table anyway.... 1/4-20 is a size #7 dril. Wes

  • Great to know and awesome addition. Thanks for the contribution! – Pᴀᴜʟsᴛᴇʀ2 Jan 13 '17 at 19:13

The correct drill size is also determined by the hardness of the material being tapped, and the "thread class" (tightness of the match between the mating threaded parts). Softer material generally gets a slightly smaller hole.

protected by Pᴀᴜʟsᴛᴇʀ2 Jan 14 '17 at 15:07

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