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Allen key sizes for bikes: the metric list and Torx

Updated: 22 October 2026

You are packing a tool for a bike that uses hex bolts, and the set in the drawer was meant for a bookcase. Allen key sizes for bikes are a short metric list, plus a check that the bolt is not Torx. This page stays on that job. Peter F. wrote it on 22 October 2026 from UK bike-shop guides and the nine amazon.co.uk listings in our [Allen key comparison](/hub/hand-tools/allen-key). We did not torque a rotor in a lab. A qualifying purchase can earn a commission at no extra cost to you. If the key is inch-sized and the bolt is metric, use the [metric or imperial Allen key](/hub/hand-tools/allen-key/sub/allen-key-metric-or-imperial) page. Pack 4 mm, 5 mm and 6 mm first.

You are packing a tool for a bike that uses hex bolts, and the set in the drawer was meant for a bookcase. Allen key sizes for bikes are a short metric list, plus a check that the bolt is not Torx.

Our top picks

AMTOVL AMTOVL 9-piece metric set

Best overall

AMTOVL AMTOVL 9-piece metric set
7.6/10about £9.49
View offer
YOIbuy PRO YOIbuy PRO 9-piece ball-end set

Rank 2

YOIbuy PRO YOIbuy PRO 9-piece ball-end set
6.2/10about £19.90
View offer
Stanley Stanley 2-69-264 folding set

Best value

Stanley Stanley 2-69-264 folding set
7.8/10about £7.42
View offer
On this page
  1. 1.The millimetre sizes on a bike
  2. 2.What the sets in this comparison cover
  3. 3.Torx is not a hex key
  4. 4.Ball end and when to stop

The millimetre sizes on a bike

A modern UK bike is metric. The sizes you turn most are 4 mm, 5 mm and 6 mm. Tredz treats 4 mm and 5 mm as the essentials and 6 mm as the next key. Add 2 mm and 3 mm for limit screws. Add 8 mm for pedals and some crank bolts. Halfords' basic ring is 2, 3, 4, 5, 6 and 8 mm at £5.99, with no 2.5 mm and no 10 mm. Park Tool's HXS-1.2 adds 2.5 mm and 10 mm. If one set serves the bike and the shed, include 8 mm and do not stop at 6 mm.

What the sets in this comparison cover

AMTOVL's 9-piece, about £9.49, and YOIbuy, about £19.90, both print 1.5 mm through 10 mm. YOIbuy is the ball-end version. AMTOVL's title says hex and one bullet says ball, so confirm the tip. Stanley, about £7.42, is 1.5 mm to 8 mm: the ride kit is there, 10 mm is not, and reviewers say the arms do not lock on a tight pedal. MULWARK reaches 8 mm and adds SAE that a modern bike does not need. G4GADGET stops at 6 mm, so the pedal key is missing, and keys fall off the ring. ELEAD does not print sizes. LOZAGU prints metric through 10 mm, and reviewers say the plastic handle snaps.
AMTOVL AMTOVL 9-piece metric set

Testsieger

AMTOVL AMTOVL 9-piece metric set
7.6/10about £9.49
View offer

Torx is not a hex key

A socket that looks almost hex may be Torx, a six-point star. A hex key cams out and rounds it. Tredz treats T20 and T25 as common, with T30 on some Scott and Bergamont bikes. road.cc points to T25 on many disc rotor bolts and T30 on some chainring bolts. A hex has flat sides. Torx has points. Forcing 5 mm into T25 is the same mistake as forcing 5/32 in into 4 mm. None of the nine sets is a Torx set, and a stray MULWARK review that mentions Torx bits is not the bullet list. If a guide quotes about 6 N·m, read the bike manual. That figure was not measured here. The bolt needs a seated Torx bit.

Ball end and when to stop

Finish a bike bolt on the straight end. Park Tool keeps the ball for access and the short end for high torque. YOIbuy's 25-degree claim does not change that. How the straight end should sit is the how to use an Allen key page. Stop when the hex is already round, when a seatpost is seized, or when the bolt is Torx and you only have hex. More force will not restore the corners, and it can crack a bonded seatpost or a frame. Metric 4, 5 and 6 mm belong in the bag, plus 8 mm if you remove pedals.

Common questions

Do I need imperial keys for a modern bike?
Not as the default. Modern bikes in this material are metric. An imperial key is for an older fastener you have identified, not for the stem bolts on a current bike. A mixed folder such as MULWARK adds SAE you will not use on that bike, and its metric side still stops at 8 mm.
Is 8 mm on the bike, and will a multitool loosen it?
8 mm is common on pedals and on some crank bolts. A multitool often includes it on a short arm, which is why it feels hopeless on a tight pedal. A loose L-key or a longer arm of the same size is the usual fix. A set that stops at 6 mm, such as the G4GADGET metric list, does not include it at all.
What is T25 if it is not a hex?
T25 is a Torx size, a six-point star, common on disc rotor bolts. T20 and T30 appear on some stems, clamps and chainring bolts, including T30 on some Scott and Bergamont bikes in the Tredz guide. It is not a 4 mm or 5 mm hex. You need a Torx bit of that size, seated fully, and the bike manual for how tight the rotor should be.
Can I use a hex key in a Torx bolt?
No. The hex cams out of the star and rounds it. After that, the correct Torx bit may not bite either. If the socket has points rather than flat sides, put the Allen key away. None of the nine sets here is sold as a Torx set, even if a stray review mentions Torx bits.
Is a ball end all right on a stem or a rotor?
You can use a ball to run a loose bolt at an angle. Do not finish a stem, a brake or a hex rotor bolt on the ball. Park Tool keeps the high-torque end straight, and leaves the ball off 8 mm and 10 mm on its workshop set. A rounded safety bolt is a shop job, not a cue to try the other end.
When should a bike shop take over a rounded bolt?
When the hex on a brake, a stem or another safety bolt is already round, or when a seatpost is seized. More force on an Allen key will not restore the corners, and it can damage the frame or the part that holds the brake. Fit a new key for the next bolt. Leave the ruined one to a shop.

Related pages

Allen key set buying guide

Allen key

Best Allen key set UK: pocket, mixed house or drawer

How to use an Allen key without rounding the socket

Metric or imperial Allen key: the sizes that round

Sources

Sources for this comparison, last checked on 22 October 2026.

  1. ISO 2936:2014, hexagon socket screw keys (retrieved 22 October 2026)
  2. HSE INDG291, providing and using work equipment safely (retrieved 22 October 2026)
  3. GOV.UK, General Product Safety Regulations 2005: Great Britain (retrieved 22 October 2026)
  4. Consumer Rights Act 2015, section 9 (retrieved 22 October 2026)
  5. Halfords, Park Tool AWS-8 ball-end hex set (retrieved 22 October 2026)
  6. Halfords basic 6-piece Allen key set (retrieved 22 October 2026)
  7. Velo, your complete guide to hex keys (retrieved 22 October 2026)
  8. RDG Tools, Bondhus ball-end hex double pack (retrieved 22 October 2026)