Treated correctly, timing belts can be better than chains. Here’s how.
Things were much simpler in the old days. At one time, vital parts of the engine were prevented from smashing together, by being synchronised with a dependable chain. Buried within the bowels of the engine, it was intended to last the life of the car and, even if worn, the driver was alerted by a tell-tale rattle long before chain failure could cause the engine’s vital internals to become violently and expensively intertwined.
Progress – back & forth
An advantage of early side-valve and pre-1960s overhead-valve (OHV) engine designs was that their cam and crankshafts sat fairly close together within the engine block, meaning that the interlinking chain’s length was kept relatively short. On some engines, such as Ford’s Cologne V4 and V6 units, solid gears were used instead of timing chains, but even they could suffer from stripped teeth as the engine bearings became worn. Most OHV engines linked the camshaft and valves together through a series of rods and rockers, hence their description of ‘push-rod’, which required tappet adjustment at every major service. Push-rod engines lasted until fairly recently, one of the most famous being Ford’s Endora-e; its lineage can be traced to the 1950s Kent unit, which ended its days powering certain 1300cc variants of the first-generation Ford Ka.
With the desire for more power, improved refinement and greater efficiency, overhead camshaft (OHC) engines replaced the push-rod’s valvegear, by placing the camshaft over the valves. This created a problem for the humble timing chain. With the camshaft positioned further away from the crankshaft, not only did it have to be longer, which dictated more elaborate tensioning mediums, but also the risk of elongation from wear (chains do not ‘stretch’) increases the chance of not only failure but the engine’s timing also being affected. The solution was to mount a toothed belt outside the engine
block, where it could not degrade from oil contact. The timing belt, or cambelt, had arrived. Compared to a chain, the belts were more efficient, quieter and cheaper for car-makers to install.
As many readers will testify, the main disadvantage with timing belts is the need for routine replacement, for which the professional labour cost alone can run easily into many hundreds of pounds. When cambelts appeared first on mainstream cars, manufacturers tended not to be bothered too much about issues that would affect a car that was likely to be out of warranty by the time replacement was required. Nevertheless, timing belts still gave them more than a few headaches. Exaggerated renewal intervals on the service schedules meant that failing belts caused catastrophic engine damage. Interestingly, this issue appears not to have gone away entirely. In the 1980s, belt change intervals tended to be quoted at between 40,000-60,000 miles, but breakage could happen well before then. Today, official belt change quotations tend to be between 100,000150,000 miles, or 10-15 years, but many experienced mechanics advise that 80,000 miles, or eight years, is a far more realistic interval. Even so, according to Andrew Vaux, Technical Training and Support Team Specialist for the Gates Corporation, a major belt manufacturer, modern timing belts may look very similar to those made in the 1960s but many unseen material changes have been made, including to the cord, jacket and tooth profile.
Realising that many fleet customers – responsible for more than 80% of new car sales in the UK – were factoring maintenance costs as part of their purchasing decisions, car-makers reacted by redesigning OHC engines to accept internal chains instead of timing belts. Except in very rare cases, there was no replacement interval, meaning that fleet car managers could trim a useful maintenance cost from their running calculations. Unfortunately, the traditional disadvantage with using long chains in an OHC application remained, not helped by modern considerations that include the quality of recycled metal, the need to reduce weight, very
lengthy oil change intervals and, of course, manufacturing costs.
The result was that the supposed fitted-for-life timing chains were either rattling, or even failing, at very low mileages. As they were not designed to be replaced, some cars (such as those fitted with BMW’S N47 and N57 engines, which locate the timing chain between the engine and transmission) require the engine to be removed just to gain access. This has become such an issue that certain car manufacturers, including the Volkswagen Group, have performed yet another volte-face, by replacing certain engines’ chains with belts. Thus, it may not be worth paying extra for a car fitted with a timing chain, because the unscheduled cost of replacing it can be far more involved and expensive, when compared to a timing belt.
Replacing timing belts and their associated parts is a fiddly task, which is why garage labour rates are so high.
Consider other parts that may be required. Pictured on the top left of this Mitsubishi engine is a balance shaft belt, which must be replaced at the same time as the main timing belt. Note also the hydraulic belt tensioner, which utilises engine oil pressure in this dry belt application (circled).
Never fit the timing belt alone. Install a kit that includes, if appropriate, a tensioner and idler roller. Depending on your car, you may need other parts – such as a water pump – if it’s driven by the timing belt.
When fitting the timing belt for the first time, the belt may need to be placed in a specific position. On this Renault F-type engine, two marks on the belt must align with the notches on both the crankshaft (pictured) and camshaft. The socket, incidentally, is holding the belt in place temporarily.
If removing the water pump, its condition will give you an idea of the cooling system’s soundness. This one was clean, due to regular coolant changes. Interestingly, this pump’s bearings were starting to stiffen after 80,000 miles of being driven by the belt, the official change interval of which was 150,000 miles.
As assessing belt wear is almost impossible, never refit a used belt. This one exhibits missing teeth, the most common failure.
Certain examples of the latest generation three-cylinder units are utilising BIO technology. Pictured is Ford’s Fox Ecoboost engine, with the timing belt shown clearly as being situated within the engine block.