Joe Bender-Zanoni pointed out
> I think the answer is 8mm X 1.25 which is a standard metric
> coarse thread! Even if 5/16X20 is right, the metric size is
> is so close it should work anyway.
Yay! Thanks Joe. I just checked with some bolts I had - 8 x 1.25 is a very common thread - on bicycles it is found on many Japanese handlebar stems, like SR and Nitto. First one I tried didn't thread in, then I noticed the first thread was banged up. Second one also no-go - also a bad thread (what's the chances of that? At my house, a sure thing apparently). Unthreaded the third bolt from inside a nut, so it was a known-good thread, and it fit perfectly! Well actually a bit BETTER than the original-equipment bolt, but I can live with that! ;)
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 oversized.
Since everything I'm looking at and measuring is grimy, and made to fairly loose tolerances, there's a chance the Williams was actually made to fit 8 x 1.25, not 5/16" 20 TPI as I said earlier. Given how close they are, we may never know unless some Williams factory documents surface.
So now I just need to find them with an appropriate head on them, thin enough to fit under the dustcaps. I'm not against lathe-turning some down that are a bit thick-headed, having a bit o' that problem meself. Allen-headed Japanese stem bolts will not work at all unfortunately, so I'm still looking, but it's looking good.
Mark Bulgier
Seattle WA USA