British Threads - UNC
UNC (Unified Coarse Thread)
Standardised in 1918
Angle of Thread 60°
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.
Both these threads are 60 degree angle.
Early Harley and Indian motorcycles used 24tpi threads on most diameters 5/16 and 3/8 UNF are both 24tpi.
7/32 x 24 UNC is an obsolete thread
| 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) |