Why it is crucial to ensure bolts are tightened to the right level

What does it mean when the mechanic says he needs to “re-torque” the bolts, after I recently replaced the car’s front suspension?

Most car components are secured to the chassis or sub-assemblies using bolts – these bolts are either paired with nuts or screwed into threaded holes.

In simple terms, when a bolt is torqued, it is tightened. A spanner is used to turn and tighten the bolt – this turning effort is called torque.

The challenge is that, relying only on the “feel” of your hand muscles, there is no way to ascertain if a bolt is tight enough or excessively tight. Either condition can cause a problem in the long run.

In some components and assemblies, having just the right amount of tightness is crucial for proper fit, durability, efficient function and perfect sealing. This is where the torque setting comes into play.

A special “spanner” known as the torque wrench is used. It has a calibrated adjustment for the setting of torque limit in Newton-metre or imperial pound-foot. When used for the final tightening of bolts, the most common type of torque wrench will indicate the preset value with a click.

Without a torque wrench, a bolt might be under-tightened, which usually leads to the mounted part coming loose. But if it is over-tightened, the bolt can shear or snap. This is due to tensile fatigue, or thermal expansion if the bolt is on an engine part.

The cylinder head in an internal combustion engine is perhaps the best example of why torque is critical. This component is mated to the engine block, an assembly that experiences high stresses and extreme temperature swings.

Each of the bolts – whether there are eight or 12 – must be tightened evenly to a specified torque. This ensures perfect fit and seal across the engine’s operating range.

Tightening torques are specified in the motor workshop manuals for components such as wheel hubs, suspension and transmission assemblies, and most parts mounted on the engine block.

It is also a recommended practice to check and re-torque bolts, especially on moving parts, after some hours of use. Additionally, when replacing broken or rusty bolts, always use the correct grade of bolt for the specified tightening torque.

Information on the various sizes and types of bolts is readily available on the internet.