What are the tolerance specs on the major diameter of imperial bolts?

Specifically, a 1/4-20 imperial bolt has a major diameter of 0.25in, but what is the tolerance on that dimension. I (and everyone I've asked) have always taken that value to be exact, but that's obviously incorrect.

  • $\begingroup$ Thread only or shank? $\endgroup$
    – Solar Mike
    Commented Sep 8, 2017 at 16:18
  • $\begingroup$ Just the major diameter on the threads. I've found a few standards docs with tolerances on unthreaded shanks, but nothing on the threaded portion. $\endgroup$
    – Diesel
    Commented Sep 8, 2017 at 16:19

1 Answer 1


Here is a link to a screw thread chart.


Note there are different tolerance classes, and also note the max is never over the nominal size.

For the example 1/4-20

class 2: 0.2408 < d < 0.2489
class 3: 0.2419 < d < 0.25
  • $\begingroup$ Cool, thanks! I find it a little interesting that the tolerances on the class two are both minus from the nominal dimension. I thought that was against ASME Y14.5 standard? $\endgroup$
    – Diesel
    Commented Sep 8, 2017 at 17:49
  • $\begingroup$ The nominal size is the minimum major diameter of the internal thread. (follow the link on that engineers edge page) The "class 2" is essentially a deliberately loose fit. $\endgroup$
    – agentp
    Commented Sep 8, 2017 at 17:52
  • $\begingroup$ RIght! it's limit dimensions not tolerance dimensions, which makes sense. The loose fit makes sense. $\endgroup$
    – Diesel
    Commented Sep 8, 2017 at 17:55

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