When do I need to replace my timing belt?

Having a timing belt break can ruin your day. It is (almost literally) a spanner in the works.

A broken belt will leave a motorist stranded by the side of the road, with only one way to get the car home or to a workshop: on the back of a tray truck.

And the damage done can cost a small fortune to fix. There is a host of mechanical components in need of repair or replacement when some parts stop rotating, but other parts don't.

While you can let the spark plugs last until the next service (perhaps) or put off replacing that air filter – not that either of those choices is recommended – replacement of the timing belt when it's due is critical. At the very least the mechanic servicing the vehicle should inspect it for wear and tear.

Manufacturers usually recommend an interval of around 100,000km or roughly five years on average for the replacement of the timing belt. After a number of unexpected breakages, Holden recommended replacement at 60,000km intervals, rather than the original interval of 120,000km, for its TS-series Astra from 1998.

Other than exceptional circumstances like that, the car's service manual outlines how frequently the belt should be replaced. If in doubt, check with the service adviser at the local dealer or after-market workshop. The service adviser will most likely volunteer the information when the part is due for renewal anyway.

Over the past few decades the timing belt has become popular with manufacturers of overhead cam engines because it requires no lubrication and is quieter and more efficient than the timing chains that preceded it. But catastrophic failures due to belt breakages mean the timing chain has made a comeback in recent years.

Basically, the belt is a loop of toothed rubber or some sort of compound material, with (steel) cables embedded for durability. It's not usually visible, being located under a cover that keeps dust and dirt out of the engine. Replacing the part requires that this cover be removed first.

As its name suggests, the part connects the crankshaft with the camshaft(s) and ensures that both turn in synchronisation with each other so that the pistons and valves can move up and down inside the cylinders, without colliding.

Take a rubber band and hold it stretched between two pens in parallel. Turn one pen and the rubber band will turn the other pen, if you're holding the pen loosely enough.

One pen is the crankshaft, which supplies power (actually rotating energy or torque) and the other pen is the camshaft, which opens and closes valves in the engine to release exhaust gases or draw air into the combustion chambers.

Not only does the belt take torque from the crankshaft to drive the camshaft(s), it does so in a precisely timed way – turning the camshaft(s) at exactly half the rotational speed of the crankshaft.

In a four-stroke engine the valves are only required to remain open for two of the strokes – intake or exhaust. For the compression (up) and power (down) strokes the valves are closed.

On the intake stroke the intake valve doesn't begin to open until the piston has begun to descend in its cylinder. And for the exhaust stroke the valve closes before the piston has reached ‘top dead centre’, leaving room for the piston at the peak of its travel. Once the drive from the crankshaft to the camshaft(s) is interrupted, the camshafts can turn independently of the crankshaft, so the valves can conflict (or 'interfere' with) the pistons.

If a belt breaks while the car is moving, the camshafts will immediately cease turning, leaving some intake or exhaust valves partially or fully open. The sudden loss of power combined with the car's forward momentum means the drive reverses direction, sending torque back to the engine through the transmission.

So pistons connected to the crankshaft by connecting rods keep rocketing up and down inside the cylinders, even as the vehicle begins to slow down. Each stroke brings the crown of the piston into contact with the partially open valves – at a relatively high impact speed.

To put this in context, take the example of a four-cylinder vehicle with a stroke of around 88mm. At a normal operating speed of 3000rpm each piston is travelling at nearly 32km/h in each direction inside the cylinder. When the pistons collide with valves stuck open in the combustion chamber the piston velocity is around the same speed as a heavy blow from a hammer.

The valve is likely to be bent out of shape by the impact and will leave gouges in the crown of the piston – if not worse. By that point the vehicle owner is facing a repair bill in the thousands instead of a few hundred dollars every five years or so.

A mechanic will likely need to replace valves and perhaps camshafts, rocker arms and a couple of pistons. Shrapnel from the valves or pistons will float around inside the combustion chamber, gouging the cylinder liners and combustion chamber, which will need honing as a consequence. Might as well replace the rings and bearings too, while the vehicle is undergoing such a major repair. The camshafts could be damaged also, and repairs will require a new head gasket and any other gaskets and seals that have to be removed to get to the inner workings.

Don't forget, the labour to remove an engine and then refit it can take a day – which is eight hours of work for about $150 per hour at a dealer. That's just to unbolt all the ancillaries, lift out the engine and then drop it back in, reconnecting everything. The actual labour required to repair the internals costs more again.

In other words, it could be cheaper just to buy a reconditioned engine (or even a replacement car!).