What Size is the 70's GM Alternator Pulley Nut?

Question:

What is the thread size of the pulley nut on GM Delco alternators from the 70s? I know they are all interchangeable, but what size is that?

Answer:

The older Delco 10DN, 10Si, and 12Si (and 15Si through DR44G) series alternators have a 17mm pulley shaft and a very odd sized nut. It looks like an M17 metric thread, but it is actually an imperial decimal thread, .668-20.