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Metric or Whitworth tap and die

Updated: 17.10.2026

The stud came off an older British lathe, a classic bike or a radiator, and the diameter looks close to M10 or to half an inch. Diameter is not the thread. A metric die that almost fits will damage the stud. This page separates ISO metric from British Standard Whitworth, British Standard Fine and British Standard Pipe. Every kit in our [tap and die set comparison](/hub/diy-supplies/tap-and-die-set) is metric. The cutting sequence, once you know the form, is on the [how to use a tap and die set](/hub/diy-supplies/tap-and-die-set/sub/tap-and-die-set-how-to-use) page. Peter F. used UK thread-form notes and the listing text from 15 October 2026, not a gauge of your bolt.

The stud came off an older British lathe, a classic bike or a radiator, and the diameter looks close to M10 or to half an inch. Diameter is not the thread. A metric die that almost fits will damage the stud.

Our top picks

Mannesmann Mannesmann 32-piece M3–M12

Best overall

Mannesmann Mannesmann 32-piece M3–M12
8.4/10about £18.92
View offer
Gunpla Gunpla 40-piece OT569

Alternative

Gunpla Gunpla 40-piece OT569
7.9/10about £25.60
View offer
Gunpla Gunpla 20-piece M3–M12

Preis-Tipp

Gunpla Gunpla 20-piece M3–M12
7.7/10about £16.98
View offer
On this page
  1. 1.Two different thread families
  2. 2.How to check the bolt you have
  3. 3.What these tap and die sets actually are
  4. 4.Which one to buy

Two different thread families

ISO metric fasteners use a 60-degree flank and a pitch in millimetres, written M6 × 1.0 or M8 × 1.25. British Standard Whitworth and British Standard Fine use a 55-degree flank and a pitch in threads per inch. Fine is shallower than Whitworth, so a shared diameter can still be the wrong pitch. British Standard Pipe is the same 55-degree family and a different job. BSPP, often marked G, is parallel. BSPT is tapered. A 1/2 inch BSP fitting is not a 1/2 inch Whitworth bolt, and it is not M12. FWB prints three drill tables. UNC is 60 degrees, like metric, but the pitch is in threads per inch. It is not Whitworth, and it is not metric.

How to check the bolt you have

Use a pitch gauge, not the spanner size. A metric leaf is marked in millimetres for a 60-degree thread. A Whitworth leaf is marked in threads per inch for 55 degrees. A true match sits in the grooves for the whole leaf. A near match walks out after a few crests. Protool's gauge notes describe that walk-out. If you only own a metric gauge, a failure to seat means stop. On a pipe fitting, see whether the thread is parallel or tapered. A caliper does not replace the gauge. BA, British Association, is another small-screw series on older instruments and some bikes, and it is not in these listings. Millimetres and 60 degrees mean metric. Threads per inch and 55 degrees mean a metric box will not cut it.
Mannesmann Mannesmann 32-piece M3–M12

Testsieger

Mannesmann Mannesmann 32-piece M3–M12
8.4/10about £18.92
View offer

What these tap and die sets actually are

Each of the twelve ASINs is a metric hand set. The small QWORK is M6 to M12. The 20-piece sets, the Mannesmann, both GreeSuit listings and the Gunpla 40-piece stay inside metric sizes. None prints a BSW, BSF, BA, UNC or BSP size. A heavy M2 to M18 case does not become a classic-bike kit. The best tap and die set page can only pick among metric boxes. If your gauge has shown threads per inch, leave this comparison. A metric die that almost starts on a Whitworth stud cuts a 60-degree crest across a 55-degree flank. You are left with a stud that fits neither nut.

Which one to buy

Buy metric when the bolt is stamped M, or when a millimetre leaf seats and the caliper agrees with a coarse size such as M8 or M10. Use the tap and die set buying guide to choose one tap or three inside that family. The Mannesmann pattern is three taps on coarse M3 to M12, and it still will not cut a Whitworth stud. Buy BSW or BSF, as the gauge decided, when the leaf is in threads per inch and the angle is 55 degrees. Buy a British Standard Pipe set for a pipe, a radiator boss or a hydraulic fitting, and choose parallel or taper to match. A 1/2 inch BSP tap is a different tool from a 1/2 inch BSW tap. Guessing with a metric die is how a usable stud becomes scrap.

Common questions

Is Whitworth the same as metric?
No. Metric threads have 60-degree flanks and a pitch in millimetres, written like M6 × 1.0. Whitworth threads have 55-degree flanks and a pitch in threads per inch. A shared diameter does not make them one thread. A metric set will not cut a Whitworth stud correctly.
How do I tell them apart on a bolt?
Seat a pitch-gauge leaf in the thread. A metric leaf is in millimetres. A Whitworth leaf is in threads per inch and made for 55 degrees. A real match stays in register. A near match walks out after a few crests. An unmarked stud is not metric until the gauge says so.
What is BSF compared with BSW?
Both are 55-degree Whitworth-family threads. British Standard Whitworth is the coarse series. British Standard Fine uses more threads per inch on a similar diameter. A BSW die will not repair a BSF bolt. Neither series is in these twelve metric listings.
Will a metric set do a half-inch pipe fitting?
Not if the fitting is British Standard Pipe. A 1/2 inch BSP thread is parallel (BSPP, often marked G) or tapered (BSPT), at 55 degrees. These sets do not list BSP sizes. A metric die that feels close will spoil the fitting.
Are UNC and Whitworth close enough?
No. UNC is a Unified inch thread with 60-degree flanks and a pitch in threads per inch. Whitworth is 55 degrees, so the angle differs. A metric die is also wrong, because the pitch is not in millimetres. It is a separate range.
What should I buy for a pre-metric British bike?
Do not buy one of these twelve sets until a gauge has named the thread. Classic British machines often carry Whitworth or British Standard Fine, and some small fasteners are BA. A metric box includes none of those. Cleaning the stud with a metric die that almost starts will ruin it.
Can I clean a Whitworth stud with a metric die that almost fits?
No. Almost means the crests are already crossing a different angle and a different pitch. A few turns leave a stud that fits neither nut. If the metric leaf will not seat, stop and get the Whitworth gauge and the matching die.

Related pages

Tap and die set for stainless steel

Tap and die set

Best tap and die set UK: three-tap set, most reviews

Tap and die set buying guide: form, pitch and taps

How to use a tap and die set: square start and oil

Sources

Public UK sources on thread form, tapping drills, work equipment and satisfactory quality. Listing prices and star counts are from the amazon.co.uk extracts dated 15 October 2026. Prices move.

  1. HSE, PUWER overview (retrieved 15 October 2026)
  2. HSE, using the right type of PPE (retrieved 15 October 2026)
  3. HSE EIS16, gloves and drilling machines (retrieved 15 October 2026)
  4. Consumer Rights Act 2015, section 9 (retrieved 15 October 2026)
  5. RoyMech, screw threads and BS 3643 (retrieved 15 October 2026)
  6. FWB, tapping and thread dimensions (retrieved 15 October 2026)
  7. Optimas, tapping sizes (retrieved 15 October 2026)
  8. Protool, thread forms (retrieved 15 October 2026)