M24 thread sizes
21 mm tapping drill · 26 mm clearance
3 mm coarse pitch · 2 mm fine · 19 mm hex key · 36 mm spanner
What M24 is used for
Large structural and civil fixings.
Every number for M24
| Major diameter | 24 mm |
| Coarse pitch | 3 mm |
| Fine pitch | 2 mm (M24×2) |
| Tapping drill, coarse | 21 mm (24 − 3 = 21) |
| Tapping drill, fine | 22 mm |
| Clearance hole, normal | 26 mm |
| Hex key (socket head) | 19 mm |
| Spanner (hex head) | 36 mm |
| Tapped depth, steel | 36.0 mm (1.5 × diameter) |
| Tapped depth, aluminium | 48.0 mm (2 × diameter) |
Where the drill size comes from
A tapping drill leaves enough material for the tap to cut a thread, and the amount it has to leave is one pitch: drill = major diameter − pitch. For M24 coarse that is 24 − 3 = 21 mm. The rule holds for the fine series too — M24×2 is drilled 22 mm.
Going slightly larger gives a shallower thread that is easier to tap and weaker; going smaller risks breaking the tap. The standard drill aims at about 75% thread engagement, which is where the strength stops improving enough to be worth the extra effort.
Nearby sizes
| Thread | Pitch | Tapping drill | Clearance | Hex key |
|---|---|---|---|---|
| M16 | 2 mm | 14 mm | 17.5 mm | 14 mm |
| M20 | 2.5 mm | 17.5 mm | 22 mm | 17 mm |
Frequently asked questions
What drill size for M24?
21 mm for a coarse thread. The rule is major diameter minus pitch: 24 − 3 = 21.
What is the clearance hole for M24?
26 mm for a normal fit under ISO 273. A close fit is about 25.6 mm and a loose fit around 27.0 mm — the clearance hole is the one in the part the bolt passes through, not the one being tapped.
What size spanner for an M24 bolt?
36 mm across the flats for a standard hex head. A socket head cap screw of the same size takes a 19 mm hex key instead.
What is the pitch of M24?
3 mm coarse, or 2 mm in the fine series (M24×2). Pitch is the distance between adjacent thread crests, so one full turn advances the bolt 3 mm.