UNF TPI

28 tpi for 1/4", 24 for 5/16" and 3/8", 20 for 7/16" & 1/2".

Unless it's UNEF which can be used on the large diameters such as on the top fork nuts where the 1/4 UNF 28 TPI is used on a much larger diameter.

Always have a set of calipers and a thread gauge.

 you can never assume that's what there is what it started out with.