Torque specifications appear throughout your workshop manual for every fastener from wheel nuts to cylinder head bolts. Using the correct torque is critical – under-torquing allows fasteners to loosen or leak, and over-torquing stretches bolts, strips threads, cracks components, and causes gasket failure. This guide explains what torque specifications mean, how to use a torque wrench correctly, and why getting it right matters.
What Is Torque?
Torque is a rotational force measured in Newton-metres (Nm) or foot-pounds (ft-lb). When you tighten a bolt, you are applying torque to stretch it slightly. This stretch (called preload) is what holds the joint together. The torque specification is calculated to achieve the correct preload for each specific fastener, material, and application. An M10 bolt into an aluminium housing requires a very different torque to the same size bolt into a cast iron engine block.
Why Specifications Vary
Torque specifications depend on the bolt size and grade (8.8, 10.9, 12.9 strength classes), the material being clamped (steel is stronger than aluminium and can withstand higher clamping force), whether the fastener is lubricated or dry (a lubricated bolt achieves higher preload at the same torque than a dry one), and the application (a cylinder head bolt needs precise stretch to seal the head gasket, while a bracket bolt just needs to be secure).
Torque-to-Yield Bolts
Many critical engine fasteners (cylinder head bolts, connecting rod bolts, main bearing cap bolts) use torque-to-yield (TTY) specifications. These bolts are tightened to a specific torque and then turned an additional specified angle (e.g., 90 degrees). This stretches the bolt into its elastic zone for maximum and consistent clamping force. TTY bolts must not be reused – once stretched to yield, they have permanently deformed and will not achieve correct preload again. Your workshop manual identifies which bolts are TTY and specifies the initial torque plus the angle turn.
Tightening Sequences
Multi-bolt components like cylinder heads, intake manifolds, and gearbox bellhousings have specific tightening sequences (usually starting from the centre and working outward in a spiral or cross pattern) and are often tightened in multiple stages (e.g., first pass to 30 Nm, second pass to 60 Nm, third pass plus 90 degrees). This ensures even clamping force across the entire mating surface and prevents distortion. Your workshop manual includes the sequence diagram and stage values for every multi-bolt component.
Common Torque Values
While you should always use the specific value from your workshop manual, some typical ranges give you a sense of scale. Wheel nuts are typically 90 to 130 Nm. Spark plugs in aluminium heads are typically 15 to 25 Nm. Sump drain plugs are typically 30 to 45 Nm. Brake calliper bolts vary widely from 25 to 115 Nm depending on the design. Exhaust manifold bolts are typically 30 to 50 Nm. These are general ranges only – always use the exact specification for your vehicle.
Using a Torque Wrench
Set the torque wrench to the specified value, apply smooth steady force, and stop as soon as it clicks (click type) or the indicator reaches the target (beam or digital type). Do not use a torque wrench to loosen fasteners – it is a precision measuring tool, not a breaker bar. Store click-type torque wrenches at their lowest setting to preserve the spring calibration.
Every torque specification, tightening sequence, and TTY bolt identification for your vehicle is in your workshop manual. MechanicMate offers PDF workshop manuals for over 960 models at mechanicmate.net/shop.
— MechanicMate . Questions or a second opinion? [email protected].