Tightening: How Tight Is Right, and Making It Stay

As the nut turns, the bolt stretches by a fraction of a millimetre. That stretch is elastic, so the bolt pulls back with a force proportional to it, and that force clamps the joint. The stretch is tiny — well under a tenth of a millimetre on a short bolt at full tension — which is why a fraction of a turn past snug makes such a difference, and why a joint that settles by even a few hundredths loses much of its clamp. A LONGER bolt is a softer spring, so it holds its tension better through settlement and thermal movement. That is a genuine design tool: where a joint keeps coming loose, a longer bolt through a spacer often fixes it where a tighter short one will not.

A torque wrench does not measure tension. It measures the twisting force you apply, and most of that force is spent on friction — under the head and in the threads — with only a fraction becoming clamping tension. Which means a dry bolt and an oiled bolt at the same torque end up at very different tensions; oiled can be a third tighter and may well be past yield. Torque figures always assume a lubrication condition, so if a specification says dry, it means dry. Use a torque wrench where a figure is specified, and tighten progressively in a criss-cross pattern on any joint with several bolts. Tightening one fully before starting the next distorts the parts and leaves the first one loose once the others pull down.

A properly tensioned bolt is already largely self-locking. Most fasteners that vibrate loose were never tight enough in the first place, and adding a lock washer to an under-tightened bolt fixes nothing. THREAD LOCKER is an anaerobic adhesive that cures in the absence of air between the threads. Medium strength comes apart with hand tools; high strength usually needs heat. It also seals the thread against corrosion, which is often the real benefit. A NYLOC nut has a nylon insert gripping the thread and is reliable and reusable a few times. A SPLIT LOCK WASHER is the traditional answer and is genuinely weak against vibration; a serrated flange or a wedge-lock pair works far better. WIRE LOCKING is fully mechanical and cannot fail chemically, which is why aircraft still use it — and it is the right answer whenever a fastener coming loose would be dangerous rather than merely annoying.