Theoretical values for estimating only. Confirm against the mill certificate or a stamped calculation before ordering or fabricating.
A bolted joint fails in one of two ways: it is too loose and the parts fret, or it is too tight and the bolt yields. Torque is how you land in between, and it is not a number you can guess.
The torque formula
T = K × d × F, where K is the nut factor, d the nominal diameter and F the preload. For steel fasteners in the as-received condition K is about 0.20; with molybdenum disulphide grease it falls to roughly 0.12.
Where the preload comes from
Preload is normally set at 70% of the bolt proof load: F = 0.7 × As × Rp0.2. As is the tensile stress area from the thread geometry, not the nominal shank area — for M20 it is 245 mm², giving about 161 kN in grade 8.8.
The K factor is the weak link
Torque converts to preload through friction, and friction varies with plating, lubricant, thread condition and the number of reuses. Published torque tables scatter by ±30% for exactly this reason. Where preload matters, specify torque plus a turn, or use a hydraulic tensioner.
Stainless bolts gall
A2 and A4 stainless fasteners seize without warning. Use an anti-seize compound, reduce the torque, and never reuse a stainless bolt that has already galled.
Frequently asked questions
What torque should I use for an M20 grade 8.8 bolt?
About 565 N·m in the as-received condition. The preload at 70% of proof load is 161 kN, and 0.20 × 20 mm × 161 kN gives 644 N·m before the dry coefficient is refined — always confirm against your own fastener supplier data.
What is the difference between 8.8 and 10.9 bolts?
Proof strength — 640 MPa against 940 MPa. A 10.9 bolt of the same size therefore takes roughly 47% more preload and torque than an 8.8.
Does lubrication change the torque?
Yes, significantly. The same preload needs about 30% less torque with molybdenum disulphide grease than dry, because more of the applied torque goes into stretching the bolt instead of overcoming friction.
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