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UNF (Unified Fine Thread)

Standardised in 1918

Angle of Thread 60o Degrees

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
inches
T P I Pitch Major Diameter Root Diameter
Internal Threads
Root Diameter
External Threads
Tapping Drill Size
number 0 80 0.0125 0.06 0.0465 0.0447 1.25 mm
(0.0492 inch)
number 1 72 0.0139 0.073 0.058 0.056 Number Drill 52
(1.6 mm)
number 2 64 0.0156 0.086 0.0691 0.0668 Number Drill 48
(1.9 mm)
number 3 56 0.0179 0.099 0.0797 0.0771 2.15 mm
(0.0846 inch)
number 4 48 0.0208 0.112 0.0894 0.0864 2.4 mm
(0.0945 inch)
number 5 44 0.0227 0.125 0.1004 0.0971 Number Drill 36
(2.7 mm)
number 6 40 0.025 0.138 0.1109 0.1073 Number Drill 32
(2.95mm)
number 8 36 0.0278 0.164 0.1339 0.1299 3.55 mm
(0.1398 inch)
number 10 32 0.0313 0.19 0.1562 0.1517 Number Drill 21
(4.1 mm)
number 12 28 0.0357 0.216 0.1773 0.1722 4.65 mm
(0.1831 inch)
1/4 28 0.0357 _ 0.2113 0.2062 5.5 mm
(0.2165 inch)
5/16 24 0.0417 _ 0.2674 0.2614 6.9 mm
(0.2717 inch)
3/8 24 0.0417 _ 0.3299 0.3239 8.5 mm
(0.3346 inch)
7/16 20 0.05 _ 0.3834 0.3762 9.9 mm
(0.3898 inch)
1/2 20 0.05 _ 0.4459 0.4387 11.4 mm
(0.4488 inch)
9/16 18 0.0556 _ 0.5024 0.4943 12.9 mm
(0.5079 inch)
5/8 18 0.0556 _ 0.5649 0.5568 14.5 mm
(0.5709 inch)
3/4 16 0.0625 _ 0.6823 0.6733 11/16 inch
(17.46 mm)
7/8 14 0.0714 _ 0.7977 0.7874 25/32 inch
(19.84 mm)
1 inch 12 0.0833 _ 0.9098 0.8978 23.25 mm
(0.9154 inch)

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