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Die stock size: the bolt number is not the holder

Updated: 2026-10-21

You have an M12 bolt, or a length of studding, and the listing in front of you talks about 38 mm, four screws, or a die that is not in the box. Die stock size is the outside diameter of the die the holder must clamp. The thread name on the fastener does not pick the stock. Products Compared compared the seven amazon.co.uk listings kept for this category on 19 October 2026, from £3.35 to £39.99, and did not run a lab test. The [die stock comparison](/hub/workshop/die-stock) prints each fit line. If you buy through a link on the comparison, we may earn a commission at no extra cost to you.

You have an M12 bolt, or a length of studding, and the listing in front of you talks about 38 mm, four screws, or a die that is not in the box. Die stock size is the outside diameter of the die the holder must clamp.

Our top picks

Unifizz Unifizz holder and 1/2-13 UNC die

Best overall

Unifizz Unifizz holder and 1/2-13 UNC die
6.5/10about £19.56
View offer
ALETO ALETO 20 mm and 25 mm conduit stock

Rank 2

ALETO ALETO 20 mm and 25 mm conduit stock
7.7/10about £39.99
View offer
BGS BGS 1891 pipe die stock set

Best value

BGS BGS 1891 pipe die stock set
6.8/10about £26.32
View offer
On this page
  1. 1.The number on the bolt is not the stock size
  2. 2.Split BS 1127 or solid DIN 223
  3. 3.Three ways amazon listings are packed
  4. 4.What to check on the product page

The number on the bolt is not the stock size

An M12 × 1.75 bolt tells you the thread you want to cut. It does not tell you how wide the die is. Accu's die and die-stock size chart sizes the holder from the die's outside diameter. On that chart a 13/16 in die is 20.64 mm across, a 1 in die is 25.40 mm, and a 1 1/2 in die is 38.10 mm. A holder near 38 mm is in that neighbourhood. None of these seven files claims to be a certified BS stock. A new thread on 12 mm bar, so an M12 nut will start, needs an M12 die of a stated outside diameter. The Unifizz fit line is 26–38 mm, MACHSWON is 20–30 mm, and the CoCud holders print 31–45 mm, 36–55 mm and 47–65 mm. A 38 mm die will not sit in the 20–30 mm holder.

Split BS 1127 or solid DIN 223

British split dies to BS 1127 use inch outside diameters and three screws. The centre screw opens the die for a lighter first cut, then the outer screws close it. Solid dies to DIN 223 use metric outside diameters and four screws, with no opening pass. Do not mix the two. Unifizz says five screws in one bullet and four in another, so you cannot guess split or solid from that page. A correct diameter is still useless if the die you need is not in the box.
Unifizz Unifizz holder and 1/2-13 UNC die

Testsieger

Unifizz Unifizz holder and 1/2-13 UNC die
6.5/10about £19.56
View offer

Three ways amazon listings are packed

The seven kept listings fall into three packs, and the words die stock set do not tell you which. Unifizz, at £19.56, includes a 1/2-13 UNC die of 38 mm outside diameter. M12–M16 on the fit line is capacity, not a second cutter. MACHSWON, at £3.35, includes a 3/8-16 UNC die and mentions M9–M11 the same way. If the nut is M12, neither die is the cutter. The CoCud listings are holders only: £14.99 for 31–45 mm, £19.39 for 36–55 mm, and £26.59 for 47–65 mm. Buy one when you have measured the die. The two larger files share one review block, both 4.6 from 17. ALETO, at £39.99, is a 20 mm and 25 mm conduit stock. BGS 1891, at £26.32, is an inch pipe ratchet. Neither is a round holder for an M12 button die.

What to check on the product page

Read the outside diameter first, in millimetres, and compare it with the die you own or the die you are about to buy. If the page only names M12 or 1/2 in, you do not yet know the stock size. Read the thread form on any included die. UNC in the box does not become metric because the fit line also prints an M range. On the CoCud rows, the sentence that the die is not included is the contents. Three screws and a split suggest the inch pattern. Four screws and a solid die suggest the metric pattern. Do not force a BS 1127 or DIN 223 label onto a listing that never uses those names. A 5.0 from eight is not a reason to skip the diameter check.

Common questions

What size die stock do I need if the bolt is M12?
You need the outside diameter of an M12 die, not the figure 12. A 38 mm die wants a holder in that band, such as the 26–38 mm fit line, and a 25 mm die wants a smaller stock. Measure the die, or buy a die whose outside diameter is printed, and match the holder to that number.
Is a 38 mm holder the same as an M12 die?
No. 38 mm is a width the die might be. M12 is the thread. Accu's chart puts a 1 1/2 in die at 38.10 mm, which is close to a 38 mm amazon holder, and that holder is still not a BS 1127 tool unless the listing says so. These listings do not say so.
Can one adjustable stock cover every metric die?
Not these round stocks. Each prints a band: 20–30 mm, 26–38 mm, 31–45 mm, 36–55 mm or 47–65 mm. A die outside that band will not clamp. There is no single holder in this set that accepts every metric outside diameter.
What is the difference between a split stock and a solid stock?
A split BS 1127 die uses inch outside diameters and three screws, and the centre screw opens it for the first cut. A solid DIN 223 die uses metric outside diameters and four screws, with no opening pass. Mixing them leaves the die crooked in the holder.
Does die stock set mean the die is included?
Sometimes, and sometimes not. Unifizz and MACHSWON include one UNC die. The three CoCud listings are holders only. ALETO and BGS are multi-piece sets for conduit and inch pipe, not for a single metric bolt die. Read the components line, not only the word set.
Will a UNC die in the box repair a metric bolt?
No. 1/2-13 UNC and 3/8-16 UNC are different threads from M12 or M10. The nut will not run. An M range printed beside the holder size is the range of dies that might fit, not a metric die packed beside the UNC one.

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Sources

Public UK sources used for this comparison. Listings were read on 19 October 2026.

  1. Accu die stocks and size chart (retrieved 19 October 2026)
  2. Protool diestocks (retrieved 19 October 2026)
  3. HSE metalworking fluids (retrieved 19 October 2026)
  4. MEICA electrical specification, BS EN 61386-1 (retrieved 19 October 2026)
  5. RoyMech pipe threads (retrieved 19 October 2026)
  6. Gas Safety (Installation and Use) Regulations 1998, regulation 3 (retrieved 19 October 2026)
  7. Gas Safe Register, home improvements (retrieved 19 October 2026)
  8. DETA 20/25 mm conduit stock (retrieved 19 October 2026)
  9. Technology Student, external threads (retrieved 19 October 2026)