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British Threads - UNF


UNF (Unified Fine Thread)

Standardised in 1918

Angle of Thread 60°

Unified Fine (UNF) and Unified Coarse (UNC).

UNF Thread Profile

American threads consist of two common and easily obtained 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 have 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 but 1/4 UNF is 28 tpi and a special 1/4 x 24 thread must be used. 7/32 x 24 UNC is an obsolete thread.

In the late 1960's, when even the U.S. was thinking of going metric, the giant BSA corporation decided it was finally time to scrap the 19th Century Whitworth-based system, and switch to American. Since they had lots of money invested in tooling, the switch wasn't made suddenly (or completely), so bikes and cars from the late '60's and later had a mix of all sorts of thread forms. Typically, engine internals (e.g. the thread on the end of a camshaft) stayed with whatever form it used to have, while simple fasteners (e.g. holding the mud guards on) switched to UNF.

diameter in
T P IPitchMajor DiameterRoot Diameter
Internal Threads
Root Diameter
External Threads
Tapping Drill Size
number 0800.01250.060.04650.04471.25 mm
(0.0492 inch)
number 1720.01390.0730.0580.056Number Drill 52
(1.6 mm)
number 2640.01560.0860.06910.0668Number Drill 48
(1.9 mm)
number 3560.01790.0990.07970.07712.15 mm
(0.0846 inch)
number 4480.02080.1120.08940.08642.4 mm
(0.0945 inch)
number 5440.02270.1250.10040.0971Number Drill 36
(2.7 mm)
number 6400.0250.1380.11090.1073Number Drill 32
number 8360.02780.1640.13390.12993.55 mm
(0.1398 inch)
number 10320.03130.190.15620.1517Number Drill 21
(4.1 mm)
number 12280.03570.2160.17730.17224.65 mm
(0.1831 inch)
1/4280.0357_0.21130.20625.5 mm
(0.2165 inch)
5/16240.0417_0.26740.26146.9 mm
(0.2717 inch)
3/8240.0417_0.32990.32398.5 mm
(0.3346 inch)
7/16200.05_0.38340.37629.9 mm
(0.3898 inch)
1/2200.05_0.44590.438711.4 mm
(0.4488 inch)
9/16180.0556_0.50240.494312.9 mm
(0.5079 inch)
5/8180.0556_0.56490.556814.5 mm
(0.5709 inch)
3/4160.0625_0.68230.673311/16 inch
(17.46 mm)
7/8140.0714_0.79770.787425/32 inch
(19.84 mm)
1 inch120.0833_0.90980.897823.25 mm
(0.9154 inch)