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Discussion Starter · #1 ·
Is 5/16 x 18tpi the same as M8 x 1.25? The M8 nuts seem to thread on with the same amount of resistance as 5/16 nuts and vice versa. The fasteners in question are the throttle body studs on a '93 Mustang 5.0. That drivetrain (mostly metric) is going into my '54 Ford Customline (all SAE).
 

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More for Less Racer
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It is fairly close but not the same.
5/16" x 18 TPI is .05556" per thread
M8 x 1.25 is .049216" per thread

5/16" diameter is .3125"
M8 diameter is .31496", nearly almost .3150"

Depending on the tolerance of the tooling used to cut the threads, you may be able to screw these two tegether where there is short amount of contact, like a nut....but threaded into a hole something like 1/2" or more deep and it could give you problems with the thread pitch by that point.
 

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Old(s) Fart
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No metric threads are "the same" as SAE. As noted, some are close and depending on the quality of the thread cut can be forced together, but the actual thread surfaces will not line up and will not carry the intended loads. Hint: the sloppier the threads, the more likely they are to go together. This should tell you something.
 
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