Rusty one... thanks. I just got my shipment of 3/8-20 BSF bolts, they thread into my old Schwinn stems and tighten down like a dream, much better than my last couple of original bolts that have some thread damage.
I can't imagine that tapping the old ones would make them fail, these old stems are pretty bullet proof unless you bend then inside out. I've got one that somebody literally twisted open so that the jaws are open about an inch and a half and the top of the stem is twisted 90 degrees sideways... but it didn't break. And, "over time?" These are 50 and 60 years old already.
Anyway - For those looking for an alternative to paying $10 to $20 for a pinch or binder bolt, if you can find someone selling it - these work!
Pros: I bought four 3/8-20 BSF bolts from Moss Motors for $17 shipped. Quick shipping, better price, and they work great.
Cons: They are black coated steel, and the hex heads are slightly wider than original Schwinn bolts, but not as thick. They will not look at all original.
The heads may polish up nice and not look too bad. I'll post pix if I do that.
If you're just fixing up a rider/beater, and the old bolts have crummy threads or rounded off head, these make a great bargain replacement.
There is no mystery. 3/8 inch 20 TPI is the size.