Threads: 24 tpi, Italian, and the day I nearly cross-threaded the bottom bracket
The bottom bracket shell on this frame is Italian-threaded, which I did not know until I had already bought the wrong replacement. Italian means 36 mm by 24 threads per inch, with both cups threading in clockwise, as opposed to the English standard of 1.37 inch by 24 tpi where the right-hand cup is left-hand threaded. I learned this by trying to thread an English cup into the shell and feeling it go in a quarter turn and then bind.
Stopping at that quarter turn is the only reason I still have a frame worth working on. A cross-threaded bottom bracket shell in a steel frame can be rescued by a shop with the right taps, but the fee and the risk were both more than I wanted. I backed the cup out, cleaned the first few threads with a brass brush, and checked with a thread gauge and a caliper. The shell measured 35.9 mm across the threads and the pitch matched 24 tpi, but the right side was clearly a right-hand thread. That combination is Italian.
Finding the correct cartridge unit took three weeks. I wanted to keep the original cup-and-cone bottom bracket, but the spindle had a pitted wear track on the right side that I could feel with a fingernail, and the right cup was worn into an oval. A sealed cartridge in the correct thread, 70 mm shell width, 111 mm spindle to match the original chainline, was the compromise. The frame is original; the bearings are not; the cranks still sit in the same place they did forty years ago.
Installation, done correctly the second time: both threads cleaned, a thin coat of anti-seize on the shell threads, the cartridge body threaded in by hand from the right side until it was flush and never once needing a tool to start, then the left cup. Torque to about 50 Nm with the splined tool and a long wrench, which is firm but not heroic. Italian bottom brackets have a reputation for the right cup loosening under pedaling, because the thread direction does not fight precession. I will check it after the first few rides. I have added a line to the notebook, in capitals, that says to measure threads before ordering anything, ever.