How tight is tight enough?

How tight is tight enough?

All mechanics, amateur and professional, are frequently faced with this question when installing a bolt into a threaded hole. It is particularly critical when one is working on a late-model car on which many engine components are likely to be made of aluminium alloy.

Aluminium has undisputed advantages, but, being relatively soft, it has an unfortunate habit of stripping threads when bolts are over tightened.

The tool that mechanics have used for generations to ensure that a bolt or plug is tightened far enough, but not too far, is a torque wrench.

It looks like an overgrown ratchet handle, with a hand grip on one end and a square drive on the other to accept sockets of various sizes. In between will be shown a scale of torque values and some means of selecting the desired value for the job at hand, as stipulated in a workshop manual or manufacturer's specification sheet.

When such a wrench is used to tighten a bolt or nut, it will give you a clear warning when the preset torque value is reached. The warning may be audible, visual or by the wrench suddenly slipping instead of continuing to transmit higher torque.

Torque is the term that engineers and physicists use for the turning effort exerted on an object to make it rotate around an axis. Thus we speak of the torque provided to the drive wheels of a car which makes them rotate to propel the car.

The concept is familiar to us all from such situations as closing a door by pushing on it to make it swing round. As a bit of experimenting with a door will show, the turning effort depends on two things: How hard we push (technically, the force exerted), and how far from the hinges we press on the door panel (technically, the distance from the axis of rotation). The angle at which we push is also important, but we shall avoid this complication by assuming we always push at right angles to the door panel.

Since the SI units for force are newtons and for distance metres, it stands to reason that torque is measured in newton metres. Thus you will see the scale on metric torque wrenches marked in Nm. An older torque wrench is often of excellent quality, and, if it hasn't been abused, will give years of faultless service.

A good torque wrench is not going to be a cheap tool, but it is really an essential item for anyone working on a car. It only has to save you from the expense and hassles of one stripped thread to pay for itself. Use it freely, for anything from cylinder head bolts to flywheel bolts to spring U-bolt nuts to wheel nuts to spark plugs ... the list is endless. Although torque wrenches are rugged, especially the older ones, they are still precision tools.