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Metric or imperial tap and die: name the form first

Updated: October 2026

The bolt in your hand looks about 8 mm across, and the set you are about to order contains only M sizes. Metric or imperial is the question to settle before that order, because a metric tap and die cuts a metric thread. You want a form name, then a yes or no on the box. The Products Compared editorial team checked the twelve amazon.co.uk listings in the [taps and dies comparison](/hub/drilling-and-accessories/taps-and-dies). Every one of them is metric. Whitworth, Unified and BSP are the reasons that stock is sometimes the wrong tool. A pitch gauge does that job if you use one. A qualifying purchase can earn a commission at no extra cost to you.

The bolt in your hand looks about 8 mm across, and the set you are about to order contains only M sizes. Metric or imperial is the question to settle before that order, because a metric tap and die cuts a metric thread.

Our top picks

Gunpla Gunpla 32-piece metric tap and die set

Best overall

Gunpla Gunpla 32-piece metric tap and die set
8.6/10about £25.98
View offer
Gunpla Gunpla 40-piece metric tap and die set

Alternative

Gunpla Gunpla 40-piece metric tap and die set
8.4/10about £25.60
View offer
GreeSuit GreeSuit 32-piece tap and die set 32-SL

Best value

GreeSuit GreeSuit 32-piece tap and die set 32-SL
8.1/10about £21.86
View offer
On this page
  1. 1.Metric is 60° and a millimetre pitch
  2. 2.Whitworth is 55° and still on spares
  3. 3.BSP is a pipe thread, not an M size
  4. 4.Classic cars mix the families
  5. 5.What to buy once you know the form

Metric is 60° and a millimetre pitch

An ISO metric thread, under BS 3643, is a 60° form with the pitch written in millimetres. Coarse M6 is M6 × 1, coarse M8 is M8 × 1.25, coarse M10 is M10 × 1.5 and coarse M12 is M12 × 1.75. Fine metric is the same 60° shape at a smaller pitch. M8 × 1.0 is a different die from M8 × 1.25. Protool’s thread-form table lists metric as 60° and millimetres, and the inch forms separately. About 8 mm will not identify the bolt. A 5/16 inch Whitworth bolt and an M8 bolt can look similar across the flats and still refuse each other’s nuts. A millimetre pitch that sits in the thread means a metric bolt. A threads-per-inch leaf that sits cleanly means you are not holding that coarse M bolt.

Whitworth is 55° and still on spares

Whitworth is a 55° thread. BS 84:2007 covers parallel Whitworth-form threads, coarse BSW and fine BSF, because spares still use them. A metric tap does not stand in for either. This page does not quote a BSW threads-per-inch table. Unified threads are 60° and sized in inches, so the angle alone will not separate them from metric. You still need millimetres or threads per inch.
Gunpla Gunpla 32-piece metric tap and die set

Testsieger

Gunpla Gunpla 32-piece metric tap and die set
8.6/10about £25.98
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BSP is a pipe thread, not an M size

A basin tail can feel like a large metric bolt because the metal is about 21 mm across. It is not an M20 thread. Stevenson Plumbing’s BSP table puts 1/2 inch BSP at about 20.99 mm and 14 threads per inch, the usual basin, kitchen and radiator-valve tail. A 3/4 inch bath tail is about 26.44 mm, still at 14 threads per inch. M20 is a metric pitch of 1.5 mm or 2.5 mm, not 14 threads per inch. Parallel BSP is BS EN ISO 228-1, marked G. The pressure-tight taper series is BS EN 10226-1. A 15 mm pipe is not a 1/2 inch BSP thread. Do not recut a gas fitting with any of these sets.

Classic cars mix the families

Hagerty’s UK guide treats metric as today’s default and lists UNF, UNC, BSF and BSW on classic British cars. A metric-only set does not cover that mix. Unified threads are 60°, like metric, but sized in inches and threads per inch. An M8 tap in a 5/16 UNF hole cuts a hybrid that belongs to neither chart. NPT is not BSP. Machine Mart lists the Clarke CHT304 with metric, UNF and BSP together, and no reliable price was found for it. If the bolt is marked M, the car repairs page is about which jobs to leave to a workshop.

What to buy once you know the form

None of the twelve listings states a Whitworth, UNF or BSP size. Fine metric is still metric. M12 × 1.5 is not 1/2 inch BSP and it is not 1/2 inch UNF. Buy the form you measured. A G 1/2 basin tail needs a BSP die. A stud that gauges as UNF needs UNF. If the gauge reads M6 × 1 or M8 × 1.25, the how to use a tap and die page is the cutting sequence. Until the gauge has given you a name, leave the hole undrilled.

Common questions

How do I tell a metric bolt from a Whitworth bolt?
Use a pitch gauge, not the size of the spanner. A millimetre pitch that fits, on a 60° form, is metric. Whitworth is 55°, and BS 84:2007 still specifies BSW and BSF because spares use them. If only a threads-per-inch leaf fits, you are not holding a coarse M bolt. About 8 mm does not separate M8 from 5/16 inch Whitworth.
Will an M20 tap recut a 1/2 inch basin tail?
It will not. Stevenson’s table puts 1/2 inch BSP at about 20.99 mm and 14 threads per inch, the usual basin, kitchen and radiator-valve tail. M20 is a 20 mm metric thread at 1.5 mm or 2.5 mm. A 3/4 inch bath tail is about 26.44 mm at 14 threads per inch. A metric die on that tail damages a thread you then have to replace.
What is the difference between BSP and the 15 mm pipe?
BSP is the thread on the tap tail or the valve. Fifteen millimetres is the outside diameter of one common copper pipe, and 22 mm is the larger pipe size. DIYnot treats those as different measurements from the BSP tail. A 15 mm compression olive is not a 1/2 inch BSP die. Buy the die for the thread, and the fitting for the pipe, as two purchases.
Do any of these twelve sets include BSP or UNF?
None of them does. None of the twelve amazon.co.uk listings lists a BSW, BSF, UNC, UNF or BSP size. The fine pairs in the Gunpla 40-piece are metric fine pitches, such as M12 × 1.5 beside M12 × 1.75. Mixed sets, sold as metric plus inch sizes, are not in this comparison. A 110-piece metric case is still a metric case.
Is NPT the same as BSP?
NPT is a US tapered pipe thread. Parallel BSP is specified in BS EN ISO 228-1 and marked G, and the pressure-tight taper series is BS EN 10226-1. The Triumph Sports Six Club forum warns against treating NPT and BSP as the same port. Do not chase a British pipe thread with an NPT die because the nominal inch size looks familiar.

Related pages

Metric tap and die set buying guide

Taps and dies sets

Best tap and die set UK: match the listing to the job

Tap and die set for car repairs: pitch and the stop list

How to use a tap and die: drill size, then the sequence

Sources

Sources from the research digest. amazon.co.uk listings in this category were captured on 15 October 2026. We did not run a laboratory test.

  1. Ultimate Handyman, Tap and Die (retrieved 15 October 2026)
  2. Grove Components, Tap Terminology (retrieved 15 October 2026)
  3. PG Merlin, The different types of taps (retrieved 15 October 2026)
  4. Protool, thread forms (retrieved 15 October 2026)
  5. RS, guide to metric bolt sizes (retrieved 15 October 2026)
  6. ISO 529:2017, short machine and hand taps (retrieved 15 October 2026)
  7. HSE, PUWER overview (retrieved 15 October 2026)
  8. Consumer Rights Act 2015, section 22 (retrieved 15 October 2026)