M18 A4-80 tightening torque — 291 N·m
192.47 mm² of thread at 600 MPa yield gives a target clamp force of 80837 N. The torque here is what it takes to produce that stretch.
Tightening torque
Roughly half the effort is lost to friction in the threads and another half under the head. Only about a tenth actually stretches the bolt. Oil the thread and the same clamp force needs a quarter less torque — and applying a dry-spec torque to an oiled bolt overtightens it by that same quarter.
- zinc-plated, as it comesK=0.22320.1 N·m
- plain dry steelK=0.2291 N·m
- lightly oiledK=0.15218.3 N·m
- waxed or moly-coatedK=0.12174.6 N·m
- Strength grade
- A4-80
- Pitch
- 2.5 mm
- Cross-section
- 192.47 mm²
- Tensile strength
- 800 MPa
- Yield strength
- 600 MPa
- Target clamp force
- 80837 N
- Force that breaks it
- 153976 N
- In kgf·m
- 29.68 kgf·m
- In lb-ft
- 214.6 lb-ft
These figures are a starting point
Whenever the manufacturer specifies a torque, that number wins. Bolts into aluminium, bolts squeezing a gasket and bolts with thread locker are all set differently from this calculation.
Nearby cells
Same bolt, other grades
Same grade, other bolts
How to read this
- Torque turns the bolt; what you want is the clamp force the stretched bolt provides.
- The slipperier the thread, the less torque the same clamp force needs — pick the condition you have.
- Clamp force is set at 70% of yield. A manufacturer figure always wins over this one.
- These are coarse threads. A fine thread has slightly more area, so its numbers differ.
Frequently asked questions
Q. What torque does a M18 A4-80 bolt take?
291 N·m dry — 29.68 kgf·m, or 214.6 lb-ft.
Q. And if the thread is oiled?
218.3 N·m. Lower friction means lower torque for the same clamp force.
Q. How hard does this bolt actually bite?
The target clamp force is 80837 N. It breaks at 153976 N, so you stop at 52% of that.
Q. How strong is grade A4-80?
800 MPa tensile, 600 MPa yield. Multiply by the 192.47 mm² of thread and you have what this bolt can hold.