I went through this almost exactly two years ago. On my 1948 9A, the thread is -- believe it or not -- 7/32-24.
Here's what I posted then:
Sent: Wednesday, April 18, 2018 23:28
To:
[email protected]
Subject: RE: [SOUTHBENDLATHE] Re: Cross slide screw thread
I have to confess that in all my 70 years of messing with nuts and bolts, I have never (before yesterday) encountered a #12-24 thread! Even though there it is on my ancient Craftsman Number Size Drill Gage. Right at the bottom, after 10-32. (And above with 12-28, 14-20, and 14-24, which I’ve also never encountered.)
So I drove to the nearby big box store and found dozens of #10-32 and 1/4-20, but NO #12-24s. However, the “real” hardware store a few miles away -- you know, the one so packed with stuff you can hardly get through the aisles – they actually had a box (which I almost spilled) of #12-24 bolts and nuts. I got a couple brass ones because the fit of the nut was better.
Back home, I tried the nut on the cross slide thread. I could get barely a quarter turn before it jammed. Then I tried the bolt on the South Bend nut. It went on 2 3/4 revolutions before it too jammed. The SB nut fits fine on the shaft, so I can only conclude that – at least on my 1948 SB 9A – the thread is, indeed, 7/32-24.
Sent: Tuesday, April 17, 2018 22:15
To:
[email protected]
Subject: Re: [SOUTHBENDLATHE] Cross slide screw thread
I measured mine, but I don’t believe what I got!
I get 0.218” over the threads and the 24 TPI gage sort of fits.. The threads are so short, it’s very hard to get an accurate measurement. 0.21875” is 7/32”. Could it possibly be 7/32-24? If so, I doubt you’ll find a matching tap!
At <https://sites.google..com/site/vintagenutsbolts/>
I found 7/32 diameter 24 TPI UNC listed under American Threads. Amazing! So maybe that’s it.
