Does anyone know the thread count for a standard 8mm collet?

I know it's not 8 x 1.0mm.

Eric,

Maybe the chart below will helpful. With all the different manufactures it’s hard to say if there is a standard. A lot of the measurements on the chart seem to match the Webster-Whitcomb.

RR

Thanks Rich. It would seem that 40 tpi is the same thread count as .625.

I've had the same questions Eric.
I found this conversion chart:

http://www.newmantools.com/tech/pitchconversions.htm