Why Some Bolts and Nuts Don't Fit
May/June 1997
J. D. Smith
146 Jo Marie Drive San Antonio, Texas 78222
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If you have been into engine or tractor restoration for any
length of time, you have undoubtedly run into old bolts and new
nuts, or vice versa, that don't fit. They will start for a few
threads and lock up, the obvious reason being that they have a
different number of threads per inch. Andrew Mackey and Paul
Frasier ran into this problem restoring their Arrow and Eli
engines, respectively (GEM, May 1996).
For lack of a better standard, many of the screw threads used on
early day mechanical devices were strongly influenced by what the
British were using, and the fact that we did a lot of trade with
Britain and Canada.
The British screw threads, commonly referred to as Whitworth,
are found on many mechanical devices manufactured around the turn
of the century. Actually, the British Standard Whitworth (BSW) is
the coarse thread series and the British Standard Five (BSF) is the
fine series. Approximately 50 years ago, Britain, Canada and the
United States met and agreed to have screw thread
interchangeability. The result of this agreement is what is now
called Unified Screw Threads, which are now the standard with which
we are most familiar. Also prior to this agreement, we (the U.S.)
used what we called American National Threads. This terminology was
incorporated in the unified thread agreement.
Since the initial agreement, the standard has been revised many
times, but the revisions have been more toward refinement of thread
profiles and tolerances, rather than the number of threads per
inch.
In the Unified Threads there are a number of series such as:
coarse thread, fine thread, extra fine thread and an almost
infinite number of special series. For example in one catalog, taps
are available in the inch size with fifteen different threads per
inch, and these are straight threads only and do not include
tapered threads, such as pipe threads.
For our purposes we are most concerned with the coarse and fine
threads. In the following chart there is a comparison of the
present Unified Threads and the old British Standard Threads. As an
example, compare the 5/16 inch UNF and the
5/16 inch UNC and the inch BSW. It's hard
to tell the difference with the naked eye, but it sure makes a
difference if the nut and bolt do not have the same threads per
inch. You can take the guesswork out by buying a thread pitch gauge
for about $10.00.