Unified And Metric Threads Comparison

The form or profile of screw threads is as critical
to compatibility as dimensions of thread elements.

Unified Form refers to an equilateral triangular
form of thread (apex angle 60 degrees) truncated
at crest and root. The root is generally rounded
and the crest is sometimes also rounded. This form
is used for general purpose threads, both inch and
metric, and is the most extensively used of all
forms.

The Unified American Standard incorporates two
basic thread series designated UNC (coarse)
and UNF (fine). This form segregates external
dimensions (bolt) with a suffix "A" and internal
dimensions (nut) with a suffix "B" after the
numeral that designates class of fit. (for example
2A (bolt), 2B (nut).

The ISO metric thread and the Unified inch thread
have the same form. The two thread systems differ
only in choice of diameters and pitch lengths for
standard sizes. Thus, it is possible for metric
and inch threads to be compatible if the tolerances
for the two threads overlap.

A comparison of pitch lengths for both Unified
and ISO metric screw threads indicates that the
following seven combinations are the only ones
potentially compatible:

Threads
Inch mm
72 0.35
64 0.40
56 0.45
44 0.60
36 0.70
32 0.80
20 1.25

A comparison of pitch diameters for these pitch
lengths indicates the following six combinations
are potentially compatible:

Unified* Metric**
0.73 - 72 1.8 x 0.35
.086 - 56 2.2 x 0.45
.099 - 56 2.5 x 0.45
.164 - 36 4.0 x 0.7
.190 - 32 5.0 x 0.8
.3125 - 20 8.0 x 1.25

*UNIFIED The first number indicates major
diameter in inch unit, and the second number
indicates the number of threads per inch.
**METRIC The first number indicates major
diameter in millimeters and the second number
indicates the pitch (distance between thread
centers) in millimeters.

While it is not recommended to combine a metric
and inch thread, the combinations noted above
show that the ISO metric and unified threads
are very similar. Either series could satisfy
the needs of a general purpose screw thread
system for most fastener applications.

There are 319 thread sizes listed in the U.S.
standards for Unified Threads covering a range
in diameters from 0.06 in (1.5 mm) to 6 in (152
mm) and pitches from 80 to 4 threads per inch,
(0.3175 to 6.35 mm pitch). Each thread size is
manufactured in at least two classes of fit.
The metric series of general purpose screw
threads incluse 332 thread sizes covering a
range from 1 to 300 mm in diameter and 0.2 to
6 mm in pitch. Each size may be manufactured
in a number of tolerance grades.