12-27-94

The Whitworth Screw Thread System

by Robert E. Reilly, P.E.

Did you ever start to do some mechanical work on your Jaguar and discover that none of your wrenches seem to quite fit on the bolt head? A friend of mine recently bought a pre-war SS Jaguar, and he made that comment to me. I then introduced him to the Whitworth wrench, of which he had never before heard. These tools with the strange and incomprehensible markings were once common enough that even Sears Roebuck and Snap-On carried them. Now they are getting harder to find as people's memories of what they were all about fade into the mists of outdated engineering. Many owners of earlier Jaguars are to varying extents familiar with the Whitworth screw thread system, but some are not. Perhaps some other readers will find a refresher course in the Whitworth system helpful and interesting.

Before the mid-nineteenth century, nuts and bolts were individually hand-made, specifically matched and were generally not interchangeable. Serious efforts to standardize screw threads began in 1841, when Sir Joseph Whitworth proposed a standard screw thread form based on a constant thread angle of 55 degrees. This became known as the Whitworth thread, and gained acceptance in British industry. Outside diameters of bolts began at 1/8" and increased by fractional inch increments, with a whole number of threads per inch specified for each diameter. The peak and root of the thread are rounded to a radius of r=0.1373 x pitch. The depth of the thread is d=0.6403 x pitch. Pitch is the number of inches per thread, or the inverse of threads per inch.

About the same time, an American named William Sellers developed a screw thread system based on a 60 degree thread angle, which was originally called the Sellers thread, then the United States Standard or USS thread, and finally in 1948 the Unified National Series, including UNC (coarse), UNF (fine), and UNEF (extra-fine). The UNF series is sometimes called SAE (for Society of Automotive Engineers) or ANF (for American National Fine). These threads have flat peaks and roots, with the depth of the thread being d=0.649519 x pitch.

The Germans, Swiss and French each developed their own metric screw thread forms. The metric world eventually agreed in 1898 on the Systeme Internationale (SI) metric thread series, with a 60 degree thread angle, but the watchmakers and optical measuring instrument makers each still have their own special threads.

The British Standard Whitworth (BSW), British Standard Fine (BSF), British Standard Parallel Pipe (BSPP), and British Standard Tapered Pipe (BSTP) are all thread series based on the Whitworth screw thread form. British Association (BA) screw threads have a thread angle of 47-1/2 degrees and are based on the Swiss Thury thread. Screw threads in these series are found on all older British vehicles. BSF is commonly used on engines and drive train components and body fasteners, BA is used to attach small clips and electrical parts, BSPP is found on the banjo bolts of water pumps and SU carbs and fuel pumps, but the Jaguar parts catalogues call out ANF sizes for many large suspension fasteners.

In 1965 the British Standards Institution approved a policy statement urging British industry to regard BSW, BSF, and BA as obsolescent, to be gradually replaced by International Standards Organization (ISO) metric thread.

For some reason now lost in obscurity, the distance across the flats of Whitworth hex nuts and bolt heads is not nice sensible fractions of an inch, nor is it millimeters, nor does it follow any discernable pattern such as a percentage of the bolt diameter. You are supposed to use special Whitworth sized wrenches. The sizes stamped on Whitworth wrenches refer not to the distance between the flats, but to the diameters of the nuts and bolts they are intended to fit. In practice it turns out you can get away with using metric wrenches in most cases.

It is very important to realize that it is not safe to repair damaged Whitworth threads with UNF taps and dies, or to mix and fit BSF and UNF fasteners together. The pitch or number of threads per inch is the most obvious difference, but more important and potentially disastrous is the reduction in surface contact area between the threads, which can lead to binding when torquing, loosening in vibration or complete failure under load. BSW and UNC sizes in most cases have the same number of threads per inch, so it is very possible to get these mixed as well. If you have a ruined fastener, especially a stud or nut on the engine or in the suspension, it is worth the trouble to get the right one rather than take a chance on something else getting ruined. One source in the U.S. for British fasteners and tools is Metric & Multistandard Components Corp. in Hawthorne, NY, (Tel) 914/769-5020, with branches in Burr Ridge, IL and Dallas, TX.

Robert E. Reilly, P.E.

Reference Bibleography

1. Kent's Mechanical Engineer's Handbook, 11th ed. (1936), Wiley

2. Machinery's Handbook, 14th ed. (1951) and 23rd ed. (1988), Industrial Press

3. Marks' Standard Handbook for Mechanical Engineers, 8th ed. (1978) McGraw-Hill

4. Standard Handbook of Fastening and Joining, (1989) McGraw-Hill

5. Tool Engineer's Handbook, 1st ed. (1949) McGraw-Hill

www.metricmcc.com

Author's Note:

After this article was published in the Classic Jaguar Association newsletter, another member from the U.K. responded by letter and stated that the hex sizes were originally governed by the commercially available steel hex bar stock sizes, in the days before automated screw machines, when nuts and bolts were cut from hex bar stock.

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Table of British Screw Threads

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British Thread Nominal Size/ No. Threads Hex Head Width comment

Designation Outside Diameter per Inch Across Flats

1/8 BSW 0.125" 40

3/16 BSW 0.1875 24

1/4 BSW 0.25 20 0.438 - 0.445"

5/16 BSW 0.3125 18 0.518 - 0.525

3/8 BSW 0.375 16 0.592 - 0.600

7/16 BSW 0.4375 14 0.702 - 0.710

1/2 BSW 0.5 12 0.812 - 0.820

9/16 BSW 0.5625 12 0.912 - 0.920

5/8 BSW 0.625 11 1.000 - 1.010

3/4 BSW 0.75 10 1.190 - 1.200

7/8 BSW 0.875 9 1.288 - 1.300

1" BSW 1.0" 8 1.468 - 1.480

3/16 BSF 0.1875" 32 -.340”

7/32 BSF 0.2187 28 non-preferred

1/4 BSF 0.25 26 0.438 - 0.445"

9/32 BSF 0.2812 26 non-preferred

5/16 BSF 0.3125 22 0.518 - 0.525

3/8 BSF 0.375 20 0.592 - 0.600

7/16 BSF 0.4375 18 0.702 - 0.710

1/2 BSF 0.5 16 0.812 - 0.820

9/16 BSF 0.5625 16 0.912 - 0.920

5/8 BSF 0.625 14 1.000 - 1.010

3/4 BSF 0.75 12 1.190 - 1.200

7/8 BSF 0.875 11 1.288 - 1.300

1" BSF 1.0" 10 1.468 - 1.480

0 BA 6.0 mm, 0.2362" 25.4

1 BA 5.3 mm, 0.2087" 28.2

2 BA 4.7 mm, 0.185" 31.4

3 BA 4.1 mm, 0.1614" 34.8

4 BA 3.6 mm, 0.1417" 38.5

5 BA 3.2 mm, 0.126" 43

6 BA 2.8 mm, 0.1102" 47.9

7 BA 2.5 mm, 0.0984" 52.9

8 BA 2.2 mm, 0.0866" 59.1

9 BA 1.9 mm, 0.0748" 65.1

10 BA 1.7 mm, 0.0669" 72.6

1/16 BSPP 0.3041" 28

1/8 BSPP 0.383" 28

1/4 BSPP 0.518 19

3/8 BSPP 0.656 19

1/2 BSPP 0.825 14

5/8 BSPP 0.902 14 non-preferred

3/4 BSPP 1.041 14

7/8 BSPP 1.189 14 non-preferred

1" BSPP 1.309 11

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