How tight should a bolt be, and how do I avoid over-tightening it?

The intuitive model of a bolt as something you do up as hard as you can is wrong in a way that causes real damage. A properly tightened bolt is elastically stretched and holds the joint by that tension. Past a certain point you are not adding clamping force, you are permanently deforming either the bolt or the material under it — and a bolt that has yielded is weaker than one that has never been tightened hard.

Step by step

1. Understand what tightening does.Turning the nut stretches the bolt slightly. That stretch pulls the two faces together and keeps them together as the joint is loaded. Clamping force, not friction on the thread, is what holds a bolted joint.
2. Understand what under-tightening causes.The joint faces separate slightly under load, they move against each other, and that movement lets the nut back off. Most fasteners that work loose were never tight enough in the first place.
3. Understand what over-tightening causes.Either the bolt yields — stretching permanently and losing strength — or the thread strips, or the material under the head crushes so the tension is lost as the joint settles. All three feel like a very tight bolt at the time.
4. Use a torque wrench where a figure is published.Vehicle brakes and wheels, cylinder heads, motorcycle and bicycle components, appliance and machinery mountings, structural steelwork and anything carbon-fibre. In those cases the number is not advisory.
5. Judge by feel elsewhere, using the joint rather than the bolt.Tighten until the joint stops compressing — you will feel the resistance change from steady to sudden. Add a small amount beyond that and stop. The transition, not the maximum, is the point you are looking for.
6. Match the tool to the size.A long breaker bar on a small bolt makes over-tightening almost inevitable. Use a spanner in proportion to the fastener — if it takes both hands and a foot on the wall, you are past where you should be.
7. Be far more careful in soft materials.Threads in aluminium, plastic, chipboard and timber strip long before a steel bolt yields. A penny washer and a lighter hand are both worth more than extra torque.
8. Tighten a group in a pattern and in stages.On a flange, a bracket or a wheel, work diagonally across the pattern and go round twice at increasing tightness. Tightening one bolt fully first tilts the joint and the others never seat.
9. Re-check after settling.New joints, especially in timber, plastic or with gaskets, lose tension in the first days. One re-tighten shortly afterwards prevents most long-term loosening.

Tips

• An impact wrench or impact driver has no feel at all and will strip a thread in aluminium or chipboard before you hear anything change. Use it to run a fastener down, and finish by hand.
• If a bolt suddenly goes easy while you are still turning, stop immediately. That is the thread stripping or the bolt yielding, and continuing makes it unrecoverable.
• A torque wrench needs to be released to its lowest setting after use, and it needs calibrating occasionally. A wrench left wound up for years reads badly.

Common mistakes

• Tightening until it will not turn any more — That point is past yield in soft materials and close to it in steel. You have used up the bolt's strength before the joint has seen any load.
• Using a torque wrench to undo something — Most click wrenches are not designed to loosen fasteners and using them that way damages the mechanism. Use a breaker bar, then the torque wrench to tighten.

Questions people ask