October 25, 2026 · DIY & Workshop · 6 min read

Bolt Torque Explained: How Tight Is Tight Enough?

Every bolted joint is a spring. Tighten it correctly and the stretched bolt clamps the parts together for life. Under-tighten and the joint works loose; over-tighten and you stretch the bolt past its yield point — sometimes with a snap you hear through the whole workshop. That's why every torque spec exists.

What torque actually does

Turning the nut twists the bolt, and the threads convert that rotation into tension. Most of your effort — roughly 50% — goes into friction under the bolt head, another 40% into the threads, and only about 10% into actual stretching. That friction-dominated relationship is why torque is only an estimate of preload, and why lubrication changes everything.

The classic approximation for preload:

where T is torque, d is bolt diameter and K is the nut factor — about 0.2 for dry, standard threads.

Grade matters more than size

A metric 8.8 bolt yields at 800 MPa; a 12.9 bolt at 1,100 MPa. Same size, very different torque specs — a dry M10×1.5 might take about 40 N·m at grade 8.8 but around 60 N·m at grade 12.9. Applying a stronger bolt's spec to a weaker bolt is the fastest way to strip or snap it. Imperial grades follow the same logic: SAE grade 5 versus grade 8, or the marks on the hex head that tell you which you're holding.

Wet versus dry

Lubricating the threads — oil, anti-seize, thread wax — cuts the nut factor by 25–30%. Since the same torque now stretches the bolt further, the rule is: reduce the torque spec by about 20–30% when lubricating, or the bolt will be over-tensioned. The opposite applies to corroded or dirty threads, which raise friction and leave the joint under-clamped even at the printed torque.

Tightening in the real world

• Torque in stages — half, three-quarters, then final — especially on multi-bolt flanges.
• Tighten crosswise or in a star pattern so the joint seats flat.
• Re-check torque after the first heat cycle on engine and exhaust fasteners.
• For critical joints, torque-angle methods (tighten to a snug torque, then a specified additional turn) beat pure torque for consistency.

Setting up the machine that bolt belongs to? Pulley Belts: How to Calculate the Right Belt Length covers the other half of mechanical assembly — getting the drive right.