Really Dumb and specific Flathead Question

I have a 1948, 59A flathead. It's missing the
main crankshaft pulley and bolt. The pulley
i will replace but right now I need the bolt.
I just wanna see if it'll rotate.

Does anyone know the size and thread count?
I can't exactly carry the block into the parts
store with me? It appears to be really fine threads.

Thanks in advance.

 

5/8 18

 

Awesome, Thanks a ton!