Servicing6 min read
When should a cam belt be changed?
The low-mileage car is the one most often overdue. A belt is cheap to replace and catastrophic to ignore — and if you cannot prove yours has been done, assume it has not.
By Kaine Carter
There is no job on a car where the gap between the cost of doing it and the cost of not doing it is wider. A belt is a few hundred pounds. A belt that lets go is usually a new engine, and on an ordinary car that means the car is finished.
What the belt does, and why failure is terminal
The cam belt keeps the top of the engine in time with the bottom — the valves opening and closing in step with the pistons coming up and down. Everything depends on that timing staying exact.
Most modern engines are what is called an interference design, which means that at certain points in their travel the valves and the pistons pass through the same space at different moments. Take the timing away and they arrive there at the same moment instead. Valves bend, pistons are damaged, and the cylinder head usually comes off with them.
That is why it is never a roadside repair, and why the belt is not really what you are paying to protect. The engine is.
Age or mileage, and why age catches people out
Every manufacturer publishes both a mileage figure and a time figure for their belts, and the instruction is always whichever comes first. People remember the mileage and forget the years.
A car covering 5,000 miles a year will reach its age interval long before its mileage one. The rubber degrades whether the car is used or not, and the tensioners and idler pulleys it runs on are ageing at the same rate. A garaged low-mileage car is not the safe one in this respect — it is frequently the most overdue vehicle on the street.
Worse, a worn belt is very difficult to inspect. As it degrades it can shear, the rubber coming away in strips, and by the time that is visible you are already well past the point where anyone sensible would drive it.
Does your car even have one?
Plenty of engines use a timing chain instead. A chain runs in oil and is generally designed to last the life of the engine with no scheduled replacement, so if yours is a chain there is no interval to worry about and nothing to book.
You cannot tell from the badge — the same model can be belt on one engine and chain on another. Send us the registration and we will tell you which yours is, and if it is a chain we will say so and that is the end of it.
Why the water pump goes at the same time
On most belt-driven engines the water pump is turned by the same belt. Reaching it means the same strip-down of the front of the engine — the same hours of labour you have just paid for.
If the pump fails eighteen months later you pay that labour bill a second time, and a seizing pump can take the new belt with it on the way out. It is a small part cost now against a large labour cost later, which is why we replace the pump, the tensioners and the idler pulleys as part of the job wherever the pump is belt driven.
Buying a used car? Ask the belt question first
It is the single most valuable thing to establish before you hand money over, and it is routinely skipped.
- Ask for the invoice, not the seller's word. "I'm sure it's been done" is not evidence, and the seller will not be there when it fails.
- Check the date on that invoice as well as the mileage. A belt done in 2018 at 40,000 miles is overdue on years even if the car has barely moved since.
- Work out the interval for that exact engine rather than for the model in general. They differ.
- If there is no proof, price the belt into what you offer. On a car with an unknown history it is not optional work.
Why there is no menu price
Because the labour varies more than on almost any other job. Some engines give you access in three hours. Others need the front of the car dismantled before you can see the belt at all, and that is most of a day's work before a single part is fitted.
We quote from the registration, in full, before anything starts — belt, tensioners, idlers, the water pump where it is belt driven, coolant drained and refilled, and 12 months on parts and labour. The figure you are given is the figure you pay.
If you are not sure where yours stands, send us the registration. We will tell you the interval for your exact engine, whether it has a belt at all, and whether you have time in hand. We service cars across Milton Keynes, Bletchley, Newport Pagnell, Buckingham and Leighton Buzzard.
