Last summer, good buddy Sam FitzSimmons, who seems to able to find anything, told me he found a 10-tooth, inch-pitch cog that I really wanted to get an old-man's reasonable gear on my '38 Paramount (which has a beautiful 26 front that I didn't want to replace).
It arrived. Beautiful! In fact, it was NOS, as far as I could tell. There was only one problem: it wouldn't thread on to any hub I tried. Turned out there was a reason that it survived maybe 50 years NOS: no one could screw it onto a hub. Or even a BB cup (same threading). The the inside diameter of the treading was just too small. Well, there was an old, dull, BB chasing tap lying around, and some valve grinding compound, so I mixed one part of each, and one part elbow grease, and a couple of hours and a beer later the steel cog fits comfortably on the aluminum Paramount hub. I suspect the tap is even duller now, but life happens...
Now, please, I'm not blaming Sam, in any sense at all. I never would have suspected undersize threading, and wouldn't have checked for it before selling an NOS cog. I'm really happy that Sam found it, and sold it to me for a most reasonable price. It's just that these old bikes and their parts do yield the occasional interesting opportunity to try something new. And a decent yarn to share with fellow "addicts."
Thanks also to the list member whose post a month or so ago reminded me of the semi-constructive uses of grinding compound.