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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.

Pᴀᴜʟsᴛᴇʀ2
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3 Answers3

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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

Pᴀᴜʟsᴛᴇʀ2
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vini_i
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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

Wes Johnston
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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.

Dave
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