Here are the parts. The coupling, in the foreground, had broken half of the spring-steel plates that are used to transmit rotation from the generator to the waterpump... or something like that. I never saw the entire assembly, but this was all I needed to get it fixed up.
After taking a bunch of measurements, I determined that it was a 7/8-20 UNEF left hand thread. The male threads also had a reduced major diameter, and it may have been tapered slightly - it was difficult to tell since everything was so mangled; the original nut was split!
The first order of business was to create a plug gauge for the threads. Since the male threads weren't in great shape, I decided to make a gauge so I could check the nut. This is my first time running single-point threads on my old Jet lathe, but it turned out alright!
The surface finish isn't great, but I think it's still better than the 1931 originals!
Perfect fit.
Next up was the nut itself. I bought a piece of 4140 alloy steel 27mm hex stock, which is very close to 1-1/16" hex, and prepped it with a bore and undercut. Again not the most beautiful surface finish, but I was happy with the results.
And here is the finished nut! This face clamps the gear, so it's nice and flat and concentric to the threads.
I added a chamfer on the front side, which isn't critical, and re-cut the face since it wasn't great from the part off tool.
Perfect fit!
After cleaning everything up and replacing the spring-steel blades, here is the aftermath. I'll deliver it tomorrow!
I reminded myself of the last few times cutting threads on a manual lathe. The most recent time was a pivot screw for my power steering pump bracket. The original bolt is an M11 shoulder screw with M10 threads, which certainly does not exist in stainless. I made my own using an M12 bolt as a donor. I turned the shoulder down to 11mm, re-cut the underhead area perpendicular, then I cut the M10x1.5 threads. This was on one of Woody's lathes, which is an English threaded machine, so I couldn't disengage the half-nut the entire time. I also had to get it started, feed in once the tool passed the live center, cut the thread, and feed out before it crashed into the shoulder! It was pretty crazy, but I got it done. This was around 2014ish.
Here is the finished product in the foreground, the donor screw in the center, and the original boring plated steel OEM bolt that I copied.
The time prior to that was an M12x1.5 thread on the Koni strut inserts I shortened for my hydraulic suspension setup, way back in 2007. Again I was cutting a metric thread on an English machine and couldn't disengage the half-nut, plus I had the added complexity of matching the lead of the existing thread! Again it turned out alright, but it was certainly terrifying at the time!
I need to come up with some easier lathe-threading projects next time!
