Whitworth to Metric Thread Adapters — the Conversion No Catalogue Stocks

A machine threaded in BSW or BSF meets a modern part threaded in metric, and nothing joins them — because the 55° Whitworth form and the 60° metric form were never made to mate, and nobody sells the bridge. We machine it: adapters, insert bushings and two-standard studs, one piece up. Send the two callouts →

Why Whitworth and Metric Never Mate

Whitworth is the world's oldest standardised thread — a 55° flank angle with rounded crests and roots, laid down in 1841 and used on British machinery for well over a century. ISO metric, like the American unified series, uses a 60° angle with flattened crests. The two forms differ in angle, in crest shape and in how depth relates to pitch.

The consequence is absolute: even where diameters look identical, a metric bolt in a Whitworth hole bears only on the flank tips. It gauges wrong, carries a fraction of the intended load, and each tightening cycle shaves a little more off both threads. The conversion cannot be faked with a near-miss fastener — it has to be a part that carries a true Whitworth form on one end and a true metric form on the other, each cut and gauged to its own standard. That is a single-point turning job, which is why it is our kind of part.

The Conversion Nobody Stocks

Search for a BSW-to-metric adapter and the results are thread charts, standards histories and forum threads — not products. Even Whitworth taps and dies are specialist items outside the UK; catalogue adapter ranges convert between the current standards (metric, UN, BSP, NPT) and stop there. Whitworth sits on the wrong side of that line: still holding together hundreds of thousands of machines, no longer served by anyone's stock.

That is the entire reason this page exists. The part is not difficult — it is an ordinary turned adapter — it simply belongs to a standard the catalogues retired before the machines did.

BSW Sizes and Their Dangerous Metric Neighbours

The table shows why the near-misses are so tempting — and why none of them work. Pitches are converted from TPI for comparison:

BSF follows the same 55° form in a finer pitch series — a 1/2 BSF runs 16 TPI against the BSW's 12. British machines routinely mix both series on one casting, so identify each thread separately rather than assuming the whole machine matches.

The Three Forms We Machine

• Male-female adapters — Whitworth male into the original casting, metric female for the new part (or the reverse). A wrench hex or spanner flats included as standard.
• Insert bushings — Whitworth outside to preserve the original tapped hole, metric inside for the modern sensor, fitting or fastener. The casting is never re-cut.
• Two-standard studs — the most requested form: BSW or BSF on one end for the vintage block, metric on the other for a modern nut or accessory. Each end single-point cut and gauged to its own standard.

Materials follow the job: 12L14 or 4140 steel for general work, 304/316L where corrosion matters, brass for instrument and plumbing-adjacent work. If the original was a plain steel casting, we will say so rather than sell you stainless you don't need.

Where the Boundary of This Page Runs

Who Orders These

Almost everyone arriving at this page is keeping something alive: pre-1970s British machine tools — lathes, mills and shapers whose castings are all Whitworth; British motorcycles and cars of the BSA, Triumph, Norton and Series Land Rover era; textile, printing and agricultural machinery that Britain exported worldwide and that now runs far from any Whitworth supply; and workshops fitting modern sensors, DROs or tooling to vintage iron — the exact moment a 55° world has to meet a 60° part, and the exact part nobody sells.

Related Pages

Where the Whitworth Catalogue Stops

Whitworth is out of production, so almost nothing here is a shelf item — that is the whole reason this page exists.

What you need	In the catalogue?	How we machine it
A modern metric fastener for a metric machine	Stocked	If both ends are metric, buy it off the shelf — you do not need us for a standard part.
BSW or BSF against a metric thread	No	The 55° Whitworth form has to be cut, not approximated with the nearest metric size.
A metric size that looks like a Whitworth one	No	Close major diameters are the trap — they start, then bind and strip. We cut the real form.
BA or CEI on either end	No	Cycle and instrument threads reproduced from the part you send.
An adapter so an original component can stay in service	No	One adapter instead of re-tapping the casting and losing originality.
A thread measured from a worn or damaged original	No	Dimensions taken from the unworn section, then cut to nominal.
Left-hand Whitworth	No	Turnbuckles and older machine tools. Cut like any other pitch.

Thread families above are the ones this page already covers.

One Whitworth job has its own page: vintage lathe headstocks. A 55° spindle nose no chuck maker's chart lists is machined as a wood lathe spindle thread adapter — with the faced register a chuck actually runs true on.

Frequently asked

Why won't a metric bolt fit my Whitworth-threaded hole?

The two standards use different thread forms. Whitworth (BSW and BSF) has a 55-degree flank angle with rounded crests and roots; ISO metric has a 60-degree angle with flattened crests. Even where the diameter and pitch look close, the flanks only touch at their tips — the joint gauges wrong, carries a fraction of its rated load, and damages both threads. The conversion has to be made by a part that carries a genuine Whitworth form on one end and a genuine metric form on the other.

Can I force a 1/2 UNC or M12 bolt into a 1/2 BSW hole?

It may start, and that is the trap. 1/2 BSW is 12 threads per inch and 1/2 UNC is 13 — close enough to engage a few turns before binding, far enough apart to strip the hole under torque. The metric lookalikes behave the same way. On a machine you care about, forcing a near-miss thread converts a missing-adapter problem into a damaged-casting problem.

What is the difference between BSW and BSF?

Same 55-degree Whitworth form, different pitch series. BSW is the coarse series and BSF the fine one — a 1/2 inch BSF runs 16 threads per inch against 12 for BSW. British machinery commonly mixes both on one machine, so identify each thread separately rather than assuming. Send the part or a rubbing with a rule in shot if you are unsure; identifying it is our job.

Can you make a stud with Whitworth on one end and metric on the other?

Yes — that two-standard stud is the most requested form on this page. The Whitworth end goes into the original casting, the metric end takes a modern nut, sensor or accessory. Each end is single-point cut to its own standard and gauged separately. Insert bushings work the other way around: Whitworth outside to preserve the casting, metric inside for the new part.

Do you also cover BA and cycle (BSC) threads?

Yes, but know what you have first. BA threads use a 47.5-degree metric-based form and live on instruments and electrical gear — a different standard again, not Whitworth. British Standard Cycle uses a 60-degree form at 26 TPI on bicycles and older motorcycles. We machine adapters for both, but the identification matters because the three families look alike in a photo and share no gauging.

What do you need from me to quote a Whitworth adapter?

The size and series of the Whitworth end (for example 3/8 BSW or 1/2 BSF), the metric callout for the other end, the form you need — adapter, bushing or stud — plus length, engagement and material. No CAD is needed. If you cannot identify the thread, send the mating part or clear photos with a rule; a thread that old is often easier to measure than to name.