Hey, thanks again for all the replies,
I'll explain a bit more why I'm asking.
My lathe is a secondhand Nova 3000 and it came with a supernova chuck (so the guy said, though I don't think there's anything written on it). This chuck comes with an insert which just happens to fit on my other lathe, a Record, so it must be 3/4 x 16tpi. If I take the insert out then the chuck fits onto the Nova lathe, which according to what I've read should be impossible, because as Tasmanian Devil says the chuck thread should be M33 x 3.5, and as Olwen says the headstock thread should be 1 1/4" x 8 TPI
This is important because I need to know the real headstock thread size (for buying more accessories of the right size), otherwise it wouldn't matter. So how do you measure the thread size?
It has just occurred to me that 1 1/4 x 8 tpi and M33 x 3.5 might be close enough to each other to fit without being a proper fit? In which case I have the wrong chuck for my lathe?