Mark Buliger wrote
8mm is a TINY bit bigger than 5/16" (two and a half thousandths of an inch bigger), enough to take out some of the wiggle that the Williams bolt had. And 1.25 mm thread pitch works out to 20.3 TPI - close enough to 20 TPI that the bolt doesn't begin to bind noticeably over the half-inch or so length of engagement, despite being slightly overs
While this is correct and the bolt may actually do the job the real difference in the two bolts lie in the angle of the thread.A Whitworth thread of any size has a thread angle of 55 degrees while a Metric thread has a thread angle of 60 degrees.A properly formed new thread will have the faces of the threads matching and will bind against a thread of a different angle.Of course a worn thread may find a mate more easily and not have any problems. Just attempting to delay anyone damaging a thread inadvertantly, Geoff Duke in Melbourne Australia, with summer rain today