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UNF
(Unified Fine Thread)
Standardised
in 1918
Angle
of Thread 60o Degrees
Unified Fine (UNF)
and Unified Coarse (UNC).
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) |
