What size spanner or socket does an M27 bolt take?

Answer 41 mm

An M27 hex bolt takes 41 mm across the flats — spanner or socket, and the same for its nut. A socket-head cap screw of the same size takes 19 mm.

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How it is worked out

Spanner / socket — across the flats of the headM27 → 41 mm
Hex (Allen) key — socket-head cap screwM27 → 19 mm
The nut takes the same spanner41 mm fits the M27 hex nut too

A standard hex is about 1.5x the thread diameter across the flats; the exact figure is fixed by the standard, not the formula

Worth knowing

Spanner size (across flats): 41 mm
Socket size: The same as the spanner — 41 mm.
Hex (Allen) key: 19 mm for a socket-head cap screw of this thread.
The nut: A standard M27 hex nut takes the same 41 mm.
This is the head, not the thread: The spanner fits the head. What the bolt threads into is the tap drill, and how hard to do it up is the torque — both linked below.

Spanner and socket size — nearby sizes

M2436 mm M3046 mm M2234 mm M2030 mm M1827 mm M1624 mm M1421 mm M1218 mm M1016 mm M813 mm

The rest of the job at this size

Tap drill for M27 × 324 mm Tightening torque for M27 × 3every grade

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