Hex Nut Spec Table

In-Depth Guide

What the nut grade number means — why the nut must be stronger than the bolt

The nut grade (4, 5, 6, 8, 10, 12) is the proof-load stress divided by about 100 MPa — a grade-8 nut withstands roughly 800 MPa of proof load on its bearing face. The design rule is nut proof load ≥ bolt tensile strength, so the nut is deliberately made stronger than the bolt. The point is that under overload the bolt shank should yield in a ductile, visible way rather than the nut thread stripping (a brittle, no-warning failure). Put a grade-6 nut on an 8.8 bolt and the nut thread strips before the bolt even yields, so the joint never reaches its rated clamp load.

Thread engagement length and the correct order for a jam (half) nut

The standard Style-1 nut height (about 0.8d) is designed to develop the bolt full strength in steel base metal. When a bolt is threaded directly into an aluminum or mild-steel tapped hole with no nut, the base metal is weaker, so the engagement length must be increased to 1.5–2d (2–3d in aluminum) to keep the thread from stripping first. A jam nut (the thin ~0.5d nut) is not for standalone clamping — put the thin jam nut underneath and the main nut on top, then hold the main nut with a wrench and tighten the jam nut backwards (counter-rotate) so the two nut threads wedge against each other and lock. Placing the jam nut on top is a common mistake and gives almost no locking effect.

Choosing among five anti-loosening methods — locknut, all-metal, double nut, serration, washer

Loosening is driven by transverse slip, not by torque (the Junker mechanism), so the method must match the environment. A nylon-insert locknut is the most common but the nylon softens around 120 °C and is good for only about five reuses. For higher temperature use an all-metal prevailing-torque locknut; for heavy vibration or repeated disassembly use a serrated flange nut (teeth on the bearing face bite the mating surface); for fine length or position adjustment use a double nut; for static, low-vibration joints a spring or flat washer is enough. A flat washer alone does not stop vibration loosening, so in vibrating joints use a prevailing-torque or serrated type.

Old vs new across-flats (AF) sizes and which face is the bearing face

For the same thread, the across-flats size (AF, width across flats) can differ between old DIN/JIS and current ISO 4032 — for example M12 is 19 mm in the old standard but 18 mm now. When servicing older equipment, trusting the table alone can leave the wrench loose and round off the corners, so measure the actual nut. Style-2 (single-chamfer) nuts also have a correct orientation: the flat, unchamfered face must be the bearing face against the mating part; fitting it upside down so the chamfered edge contacts reduces the contact area and causes stress concentration and loosening. Style-1 (double-chamfer) nuts have no preferred direction.

Torque-to-tension scatter — even one grade, friction swings clamp force ±40%

About 90% of the tightening torque is consumed by friction at the nut bearing face and threads, and only about 10% actually goes into stretching the bolt as clamp force. So the same torque produces very different clamp loads depending on surface condition — between dry, zinc-plated and MoS2-lubricated surfaces the tension at a given torque varies by 30–50%. The nut factor K = 0.2 used in most torque tables assumes a dry steel surface; apply it directly to a lubricated or plated surface and the bolt can be over-tightened into yield. For clamp-critical joints, do not trust torque alone — supplement it with the turn-of-nut (angle) method or by measuring bolt elongation.