British Tools & Fasteners - UNC Thread Profile

American threads consist of two common thread patterns known as U. N.F and U.N.C.

U.N.F [national fine] is exactly the same as the old S.A.E [standard american engineers] that existed before threads were unified.

U.N.C [national coarse] roughly derived from the British Whitworth having the same pitches except 1/2 in dia where Whitworth is 12tpi and U.N.C is 13 tpi.

Early Harley and Indian motorcycles used 24tpi threads on most diameters 5/16 and 3/8 UNF are both 24tpi.

diameter in inches	T P I	Pitch	Major Diameter	Root Diameter Internal Threads	Root Diameter External Threads	Tapping Drill Size
number 1	64	0.0156	0.073	0.0561	0.0538	1.55 mm (0.610 inch)
number 2	56	0.0179	0.086	0.0667	0.0641	Number Drill 49 (1.85 mm)
number 3	48	0.0208	0.099	0.0764	0.0734	Number Drill 45 (2.1 mm)
number 4	40	0.025	0.112	0.0849	0.0813	2.35 mm (0.0925 inch)
number 5	40	0.025	0.125	0.0979	0.0943	Number Drill 37 (2.65 mm)
number 6	32	0.0313	0.138	0.1042	0.0997	Number Drill 34 (2.85 mm)
number 8	32	0.0313	0.164	0.1302	0.1257	3.5 mm (0.1339 inch)
number 10	24	0.0417	0.19	0.1449	0.1389	Number Drill 24 (3.9 mm)
number 12	24	0.0417	0.216	0.1709	0.1649	4.55 mm (0.1791 inch)
1/4	20	0.05	_	0.1959	0.1887	Number Drill 7 (5.2 mm)
5/16	18	0.0556	_	0.2524	0.2443	Letter Drill G (6.6 mm)
3/8	16	0.0625	_	0.3073	0.2983	Letter Drill O (8 mm)
7/16	14	0.0714	_	0.3602	0.3499	9.4 mm (0.3701 inch)
1/2	13	0.0769	_	0.4167	0.4056	10.8 mm (0.4252 inch)
9/16	12	0.0833	_	0.4723	0.4603	12.2 mm (0.4803 inch)
5/8	11	0.0909	_	0.5266	0.5135	17/32 inch (13.49 mm)
3/4	10	0.1	_	0.6417	0.6273	16.5 mm (0.6496 inch)
7/8	9	0.1111	_	0.7547	0.7387	49/64 inch (19.45 mm)
1 inch	8	0.125	_	0.8647	0.8466	22.25 mm (0.8760 inch)

UNC (Unified Coarse Thread)