Helicoil from McMaster-Carr 18-8 Stainless Steel Helical Inserts 91732A201 calls out a specific tap to use, but I cannot find out what thread this tap is putting into the hole for the insert. I want to call out the tapped thread on a drawing, and put the 2-56 insert in later. I cannot find what the thread should be. This would be the outside thread of the helicoil. I know the drill size is #41 = 0.0960 but what is the thread size?


  • $\begingroup$ Please check if the add link is accurate. $\endgroup$ – user7083 Oct 13 '18 at 19:02

Helicoil taps are what's known as "STI" or "Screw Thread Insert" Taps. They have the same pitch as the fastener that they're going to be used with, but a larger outer diameter, to account for the wire going into the space between.

In my personal experience, the hole callout on a drawing should indicate the size of the fastener to be inserted into that hole, for example: "M10x1.5 STI".

This matches the way that the taps themselves are labelled and sold, with the fastener thread indicated. image of coil forming tap

  • $\begingroup$ Thanks Jonathan! That makes sense. I did not realize "STI" defined the thread type. $\endgroup$ – RickH Oct 14 '18 at 1:38
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    $\begingroup$ Glad I could help! Don't forget to click the 'accept as answer' button if this answered your question - otherwise the 'community' bot auto-bumps this question in a few months' time, which buries other new questions. $\endgroup$ – Jonathan R Swift Oct 14 '18 at 19:07

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