Why Some Bolts and Nuts Don't Fit

Content Tools

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.

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.

Just think, when our grandchildren take up the hobby of restoring present-day machinery, they will also have to contend with metric threads.

If any of you purists are interested in obtaining taps and dies of these now uncommon threads, one source is Rex Supply Company, Houston, Texas (713) 222-2251, with which my only connection is being a customer. Additional information on screw thread systems is available in the Machinery Handbook.

Screw Thread Comparison Chart

Threads per inch:

SizeUnifiedBritishStd
InchesUNCUNFBSWBSF

?

20

28

20

26

5/16

18

24

18

22

3/8

16

24

16

20

7/16

14

20

14

18

?

13

20

12

16

9/16

12

18

12

16

5/8

11

18

11

14

?

10

16

10

12

7/8

9

14

9

11

1 inch

8

12

8

10