Feature
Torque, for People Who Build Things: What the Spec Means and When Feel Wins
Rowan Blake · · 2 min
A torque spec is a proxy: what you actually want is clamp force — the bolt stretched slightly, holding the joint like a spring. Torque is just the measurable stand-in, and about 85–90% of it disappears into friction at the threads and under the head. Understanding that one fact explains every rule that follows.
Baseline values for the shop
For dry, clean, coarse-thread fasteners in steel — the scratch builder’s default — reasonable working targets:
| Size (grade 8.8 / class) | Torque |
|---|---|
| M5 | ~5 Nm (4 ft-lb) |
| M6 | ~9 Nm (7 ft-lb) |
| M8 | ~22 Nm (16 ft-lb) |
| M10 | ~44 Nm (32 ft-lb) |
| 1/4”-20 Gr5 | ~9 ft-lb |
| 5/16”-18 Gr5 | ~18 ft-lb |
| 3/8”-16 Gr5 | ~33 ft-lb |
Manufacturer specs override tables; tables override guessing; guessing overrides nothing.
The friction asterisk
Lubricated threads change everything. Oil or anti-seize cuts friction, so the same torque produces far more clamp — commonly 25–35% more — which is how “I torqued it to spec” snaps studs. Specs are dry unless stated; if you lube, reduce accordingly or find the wet spec. Thread condition matters the same way: chase dirty threads before trusting any number through them.
Sequence and stages
Multi-bolt joints (flanges, covers, anything gasketed) tighten in a cross/star pattern, in stages — half torque, then three-quarters, then full. One bolt taken straight to spec cocks the part, and a cocked part measures “torqued” while clamping nothing evenly. Aluminum and plastic add their own rule: the material yields before the bolt does, so specs are lower and a torque wrench earns its keep most exactly where parts are softest.
When feel is legitimate
Every builder eventually calibrates hands against the wrench — small fasteners especially, where “snug plus a quarter turn” is a real method (it approximates turn-of-nut, which controls stretch directly). Feel is fine where failure is annoying. Where failure is dangerous — spindles, brakes, anything rotating fast near your body — the wrench, the spec, and a witness mark. The independent part of scratch building doesn’t extend to independent physics.