Custom Whitworth Bolts and Obsolete Thread Fasteners

Who needs custom Whitworth bolts, and why?

The buyer is usually keeping something old running or putting it back together: a British car or motorcycle, a machine tool, an instrument, a piece of American equipment from before the Unified standard. The machine still takes the thread it was built with, and the standards body expects it to. BSI revised BS 84, the Whitworth standard, “on the basis that Whitworth threads will undoubtedly continue to be used for many years, especially on replacements or spare parts” (source 1). Wikipedia says BSW “remains in use in restoration, vintage machinery, and certain legacy industries” (source 2). The part quoted here is that thread on a bolt at the length, thread length, head or material the machine needs. The general case, in current threads, is on the custom fasteners page.

The American threads are older than most buyers expect. The United States Standard thread, also called the Sellers thread, “is no longer supported”; it and the SAE fastener standard were incorporated into the Unified Thread Standard, and UNC and UNF are the names now given to fasteners that would once have been USS and SAE (source 3). J.C. Blanchard, writing in Rust Magazine, dates the change to 1948, when America, Britain and Canada adopted the Unified standard (source 4). He also notes that numbered screw sizes above #12 are obsolete, along with small fractional sizes such as 7/32-24 and 3/32-48 (source 4). Wikipedia adds that a UNC thread “is mechanically interchangeable with a USS thread of the same diameter,” though tolerances and other details differ (source 3), so state on the print which name your drawing uses.

Which older threads are quoted, and how do they differ?

The threads quoted are BSW, BSF, BA and pre-unified American sizes: USS, old SAE and numbered sizes above #12. The table sets out how they differ, to help you name yours. Whatever the series, the fastener is quoted to the thread callout on your print.

Cycle thread is quoted to the print as well: British Standard Cycle (BSC), which replaced the Cycle Engineers’ Institute (CEI) standard, is the 60° thread used on British bicycles and motorcycles, where Whitworth is 55° (source 2).

How do BSW and BSF sizes compare with UNC and UNF?

Whitworth and Unified threads read alike on a size chart, but they are different threads. The Unified angle is 60° with flattened crests, and Whitworth is 55° with rounded crests (source 2). BSW and UNC share a pitch at most common sizes, but not at 1/2 in, which is 12 TPI in BSW and 13 in UNC (sources 2, 8), and a shared pitch does not make them interchangeable, because the angles differ (source 8). BSF is a finer pitch than BSW on the same 55° form (source 5), and its pitch differs from UNF at every size where the table below lists both. Name the series on the print, not only the diameter and a pitch.

The table gives threads per inch for the common sizes, 1/8 in to 1 in. BS 84 is the standard itself (source 1) and is not reproduced here; the counts are from the Wikipedia tables.

How do BA sizes run?

BA sizes are metric-based. 0BA starts at 6.0 mm diameter and 1.0 mm pitch, and each higher BA number has 0.9 times the pitch of the size before it (source 6). The angle is 47.5°, so a BA fastener does not mate properly with a Whitworth or metric one even where the diameter and pitch look close (source 6). Sizes are defined from 0BA to 22BA, and the even numbers are much more common (source 6); BS 93 covers 0 to 16 (source 7). The table gives 0BA to 10BA.

What do the current British standards cover?

Both British standards are still published. BS 84:2007 gives limits of size and tolerances for BSW and BSF threads, with tolerance classes for external and internal threads and a separate section for coated threads (source 1). BS 93:2008 does the same for BA sizes 0 to 16, with close and normal class external threads up to 10 BA, normal class from 11 BA, and a single class for internal threads (source 7). The class goes on your print. Each standard says it is intended for use with a matching gauge standard, BS 919-2 (sources 1, 7). For Unified and metric threads, custom thread gauges covers the plug and ring gauges quoted to a callout.

How do you tell which thread you have?

Start with the angle. JRC Engineering, a classic motorcycle parts seller, calls treating an unfamiliar thread as metric “the most common mistake on classic British machinery,” and gives its rule: BSW and BSF are 55°, UNC and UNF are 60°, and BA is 47.5° (source 8).

Next comes the pitch. A pitch gauge does the counting, but Wikipedia is plain that the tool “is not used as a precision measuring instrument”; it lets you “quickly categorize the thread by shape and pitch,” and “a process of trial and error may be needed if the screw is of unknown origin” (source 9). Use it for the threads per inch, take the diameter with calipers, and put both on the callout.

Do not identify the thread from the head alone. JRC says a spanner stamped with a fraction and a “W” is Whitworth, and that British fasteners from before the 1970s often carry a W on the head (source 8). But Wikipedia records that British Morris and MG engines from 1923 to 1955 “were built using metric threads with bolt heads and nuts dimensioned for Whitworth spanners and sockets” (source 2). Check the thread as well as the head.

Two more traps. BA numbers run the other way from American gage numbers: the bigger the BA number, the smaller the screw (source 4). And the British pipe threads share the 55° form, but their designations “do not refer to any thread diameter,” so a G1/2 pipe thread is 20.955 mm in diameter, not 1/2 in (source 2).

What goes on the print of a custom Whitworth bolt?

The thread callout carries the series, the diameter, the pitch or threads per inch, the hand and the class. The series matters because the diameter alone does not fix the pitch: a 1/2 in BSW has 12 TPI, and a 1/2 in BSF has 16 TPI (sources 2, 5). Add the thread length and the overall length, each as its own dimension.

The head needs the most care. Wikipedia explains that Whitworth and BSF spanner markings “refer to the bolt diameter, rather than the distance across the flats,” and that each Whitworth hexagon “was originally one size larger” than the matching BSF one (source 2). A smaller hexagon was adopted more widely during World War II “to save metal,” and “it is today common to encounter a Whitworth hexagon which does not fit the nominally correct spanner” (source 2). So do not give the head as a spanner size. Give the measured distance across the flats and the height, and the quote follows them. BA has its own set of head sizes (source 2).

A plated thread has its own limits: BS 84 has a separate section on coated BSW and BSF threads (source 1), so say on the print whether the thread limits apply before plating or after.

Turned diameters and lengths are quoted from the print. Typical machined tolerance is ±0.005 in, ±0.001 in on critical turned diameters, and down to ±0.0002 in on wire-EDM and other finished features; ground features are quoted to ±0.0001 in. A ground shank is quoted with precision grinding. Threads are quoted to the class named on the print.

What to send: a print, a thread callout, or a new sample part

A dimensioned print starts fastest, with the thread named by its old standard on it. With no print, a thread callout and a sketch with the dimensions are enough to start: series, diameter, pitch, hand, class if you know it, the two lengths, the head across the flats and its height, and the material. If you have no print and no callout, name the thread if you can, for example 3/8 in BSW or 2BA, add the dimensions you know, and the quote lists what is missing. If a worn or broken part is the only one left, send photos and a description first and the reply says what to send. Reverse engineering covers the route from a new sample part.

A new sample part is the other way in, and an unused spare works. Send a new part, not the worn one. Wear changes the sizes that matter. The sample ships back with the parts. When the drawing is made from a new sample part, the first order carries it as its own quoted line, and every reorder comes off that drawing. A fully dimensioned 2D PDF is quoted without a CAD line. Files: Any file type; STEP + PDF ideal.

Which materials and finishes are quoted?

The materials quoted are titanium, 304 and 316 stainless, brass, bronze, and steel heat treated to the hardness on the print. Other alloys and plastics quoted on request. Name the grade on the print. Each metal with its own page is linked here: titanium, 304, 316, brass and steel. Zinc plating, black oxide and heat treat are quoted with the part.

Sizes, quantities, reorders and blanket orders

Sizes run down to M1; for a larger size, send the print. A single bolt is a normal order. Ask for more than one quantity, such as 1, 25 and 100, and the quote shows each. 25 pieces is a good start. At 200, the price per part drops a lot.

Reorders at the same revision: send the last quote or PO number and it’s requoted. A year of usage on old equipment can be quoted as a blanket order and delivered in releases. Price, release schedule and lead time per release are set in the quote.

How long do they take?

Quotes come back in 24-48 hours. Rush: 1-2 weeks. Standard: 4-6 weeks (typical). Extended: 6-8 weeks, for jobs that aren't time-sensitive. Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date. Lead time starts at order approval. Put your need date in the part description and the quote answers it. For a machine that is down, see rush CNC parts.

What comes with the parts

Every part is inspected against your print before it ships. If a part doesn’t match the print, it’s remade or credited. Shipping is quoted as its own line, or ships on your carrier account.

Shouldered bolts and studs in these threads follow the same routine: see custom shoulder bolts and custom studs. Socket head screws, set screws and thumb screws are on the hub, custom screws, bolts and studs. For the turned diameters, grooves and undercuts on a fastener, see CNC turning.

Sources

Quoted passages and tabulated figures on this page come from the following, all accessed 1 October 2026. Where anything here differs from your print, the print governs. The two thread tables are adapted from the Wikipedia articles in sources 2, 5, 6 and 10, by Wikipedia contributors, licensed CC BY-SA 4.0, and are shared under the same license.

1. British Standards Institution, “BS 84:2007 Parallel screw threads of Whitworth form. Requirements,” BSI Knowledge, published 30 November 2007: knowledge.bsigroup.com/products/parallel-screw-threads-of-whitworth-form-requirements (Whitworth threads used for replacements and spare parts; BSW and BSF size ranges; tolerance classes; coated threads; the matching gauge standard).

2. Wikipedia, “British Standard Whitworth,” last edited 27 August 2026: en.wikipedia.org/wiki/British_Standard_Whitworth (55° angle and rounded crests; Unified 60° angle and the 1/2 in pitch difference; use today; spanner markings and hexagon sizes; Morris and MG; pipe thread designations; BA head sizes; BSW threads per inch and nominal diameters; British Standard Cycle, which replaced CEI, at 60° on British bicycles and motorcycles).

3. Wikipedia, “United States Standard thread,” last edited 10 August 2023: en.wikipedia.org/wiki/United_States_Standard_thread (USS and SAE folded into the Unified standard; UNC and UNF; flat crests and roots; interchangeability with UNC).

4. J.C. Blanchard, “Threads & Thread Systems,” Rust Magazine, 16 January 2018: rustmag.com/gear/2018/1/4/threads-and-thread-systems (Whitworth angle and form; the Sellers 60 degree angle; 1948 adoption of the Unified standard; obsolete sizes; BA numbering).

5. Wikipedia, “British Standard Fine,” last edited 23 April 2025: en.wikipedia.org/wiki/British_Standard_Fine (the Whitworth form at a finer pitch; use in British machinery and cars; BSF threads per inch).

6. Wikipedia, “British Association screw threads,” last edited 5 April 2026: en.wikipedia.org/wiki/British_Association_screw_threads (47.5° angle and rounded tops and bottoms; 0BA at 6 mm and the 0.9 pitch step; sizes 0BA to 22BA; where BA is used; BA pitch, diameter and threads per inch).

7. British Standards Institution, “BS 93:2008 British Association (B.A.) screw threads. Requirements,” BSI Knowledge, published 30 September 2008: knowledge.bsigroup.com/products/british-association-b-a-screw-threads-requirements (BA sizes 0 to 16; close and normal classes; the matching gauge standard).

8. Graham White and Stephen Moore, updated by JRC Engineering Inc, “Whitworth and other British Threads,” jrcengineering.com, posted 3 March 2016, jrcengineering.com/articles/whitworth-and-other-british-threads/ (the angle rule; W markings on spanners and heads; the 1/2 in BSW and UNC pitches).

9. Wikipedia, “Thread pitch gauge,” last edited 8 November 2023: en.wikipedia.org/wiki/Thread_pitch_gauge (what a pitch gauge does and does not measure).

10. Wikipedia, “Unified Thread Standard,” last edited 17 June 2026: en.wikipedia.org/wiki/Unified_Thread_Standard (UNC and UNF threads per inch by nominal size).