"DoN. Nichols" wrote

You know that 5/16 is *very* close to 8mm?

0.3125" ~= 0.315" as I said, *very* close.

18 TPI calculates out as 1.411mm -- and the M8 is probably
M8x1.5, so it will go in a few turns before wedging.

So -- the *screw* may be 5/8-18, but the threaded hole is
probably M8x1.5. Did you try purchasing a M8 screw and trying it?

The two are close enough so I would expect my "screw chkr" to
not tell the difference. You would need a proper thread pitch gauge
engaging perhaps a half inch of screw length to tell for sure.
DoN.

I store my metric taps in the drawers with the screws and use them to
check the pitch of unknown hardware. A screw picked at random from the
drawer could be mis-sorted but the taps are marked for size.

jsw