[Steeveedee]

The part that threads into the water pump flange needs 5/16" inch minimum thread, and not much more to keep from rubbing on the water pump body. That should be 5/16" fine tread, IIRC. Then add on the clutch flange thickness, about 1/4" and another 3/8" of thread for the lock washer and nut.

So, 5/16"+1/4"+3/8"= 15/16" total, minimum. 1 inch would work, but depending on how close your fan or clutch is to the radiator, if the the studs are too long you might have trouble mounting the fan with the studs in the pump, and might have to finagle it around.