# Diameter of imperial machine screws

## Main Question or Discussion Point

Dear Forum,

I am currently trying to buy some imperial machine screws and bolts online, and I am confused with how the diameter is given.

The website I am seaching (boltdepot.com) gives me these diameters but I don´t understand what is meant by i.e. "2-56"

2-56
4-40
6-32
8-32
10-24
10-32
12-24
1/4-20
5/16-18
3/8-16
1/4-28

Can anyone help me? I am specifically looking for a 5/32 inch diameter screw but this does not appear here. Anyone suggesting a different good site for buying fasteners online, preferably located in europe?

best regards
t.

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mgb_phys
Homework Helper
Below a 1/4" they are just given numbers, a #8, #10 etc - the diameter is d=0.06" + (#*0.13")
or you to look up the diameters in a machinist handbook
Above 1/4" the first number is the diameter of the bolt in inches.

In both cases the second is the number of threads per inch, so a 'quarter-20' is 0.25" diameter and a thread pitch of 0.05"

5/32 isn't a standard US size, it's almost a #7 - which is also rare.
Is this an old British part ? in Which case it might be whitworth.

Last edited:
Thanks,

It is definitely an US part.

My local store here thought it was a 5/32 and I believed them. Might even be a #7 as you say. I probably need to measure it myself....

Edit:
With respect to the list in my first post, does 2-56 then mean screw #2 with 56 threads per inch? and "4-40" is #4 with 40 tpi?

regards
t.

mgb_phys