Why Some Bolts and Nuts Don't Fit

| May/June 1997

146 Jo Marie Drive San Antonio, Texas 78222

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.