Adapter For Enlarging Lens advice

[Straterxlx]

I've picked up a cheap 50mm f2.8 El Nikkor

Am I right in assuming that this lens has a 39 mm filter thread and would need a M42 to M39 adapter to attach it to a Pentax bellows.

I don't want to buy one unless I'm certain of the thread size.

Thanks for any help.

[Sym P. le]

Pentax is M42 x 1mm and the Nikon El uses the Leica thread M39 x 26tpi. Check that adapter is intended for this use, it is quite common. There are other variations on the M42 and M39 threads.

P.S. I should add, a typical bellows setup is reverse mounted and would require the appropriate filter thread adapter depending on your lens. See comment by FotoChris below for info re. threads. Thanks.

[FotoChris]

If you want to use the enlarger lens for regular photography (up to 1:2) in forward orientation you need to mount it via the Leica M39 threading (as Sym P. le mentioned) and that's easily adapted to M42 with cheap adapter rings.
However for closeup and macro work you want to use the enlarger lens reversed, that usually gives you the best result.

However if I may correct SymP. le: Both the newer and older EL Nikkor 50mm 2.8 have a 40,5mm filter thread - the 50mm 4.0 have a 34,5mm filter thread.