What is the tolerance for through hole to match the UNC 1/4-20 tap ⌀ 6mm? (tap shaft diameter ⌀ = 6mm) I want to make the diameter of the hole such that the tap stands in the hole and doesn't fall inside the hole. At the same time, the tap must pass freely through the hole. The purpose of the hole not for threading, it will be used as guide (in hand tap guide).

  • 2
    $\begingroup$ You need a machinest, not engineers. Engineers just say how big the hole should be and where to put it. $\endgroup$
    – Tiger Guy
    Commented Apr 28 at 3:47

1 Answer 1


So, look up the tap information, and for unc 1/4 - 20 it is 6.35mm outer diameter.

You can check here: https://www.engineeringtoolbox.com/unified-screw-threads-unc-unf-d_1809.html

Which means to make sure it does not bind, and to cover tolerances of your hole drilling you need to add a bit so 6.45mm or even 6.5mm. Depends what drills you have and what equipment you use (hand drill or pillar drill etc)

The detail of what is where: enter image description here Sourced from:

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  • $\begingroup$ I just clarified my question. $\endgroup$
    – minto
    Commented Apr 28 at 10:32
  • $\begingroup$ So you don't understand "major diameter". $\endgroup$
    – Solar Mike
    Commented Apr 28 at 13:24
  • $\begingroup$ You're right, I got the sizes mixed up! Of course, what I need a major diameter measured on the root of the thread. In a UNC 1/4-20 thread, the nominal major diameter is 1/4″ (6.35 mm). $\endgroup$
    – minto
    Commented Apr 28 at 19:36
  • $\begingroup$ what drill diameter should be used before use Machine reamer Ø 6,5 G7? $\endgroup$
    – minto
    Commented May 3 at 22:16

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