Timing belt: what it is, how it works and when to change it

Timing belt: what it is, how it works and when to change it

There timing belt is an essential component for i engines combustion, since it drives the camshafts which, in turn, move the intake and exhaust valves. The movement of the timing belt must be perfectly synchronized with that of the crankshaft because otherwise the valves could touch the piston crown with disastrous consequences.


The name of this component of internal combustion engines already explains its function, which is that of set the camshafts in motion and, through them, the valves. The timing belt, which is very flexible and is wide and thin, must move the camshafts in a perfectly synchronized manner with the rotation of the crankshaft: this is why it is not as smooth as the one it drives, for example. , the alternator and the power steering pump, but it has teeth along its entire width. These teeth fit into the grooves in the shaft pulleys engine and camshafts and in this way their synchronized rotation is obtained as happens with the chain that connects the bicycle pedals to the wheel. The timing belt has teeth on one side only: the other side is smooth and generally drives the coolant pump as well as being traversed by a tensioning roller that imparts the correct tension to the belt.


In modern engines the valves are ‘overhead’ and therefore the camshafts are very distant from the crankshaft, housed in the crankcase. When engines had lower performance the overhead valves were operated by rods and rockers and therefore the camshaft was much closer to the crankshaft and could be moved by a simple pair of gears. The increase in performance and rpm has forced the displacement of the camshafts on the cylinder head in order to eliminate the inertia of the rods and the timing belt is a light and silent solution, which is also used on some motorcycles, to connect two shafts – the engine and the camshaft (s) – quite distant from each other. The gear control would in fact be too expensive while the chain, more reliable and precise, however, is noisier and more complicated to replace. Since each valve, be it intake or exhaust, moves once every two revolutions of the crankshaft, the crankshaft pulley has half the grooves of the one that turns the camshaft: the latter makes then one revolution every 2 revolutions of the crankshaft and then the valves open and close at the right time.


If the timing belt breaks the engine obviously stops because the valves stop moving and therefore the combustion cycle can no longer take place. If the distance between the piston crown and the valves is so small that the two components can touch each other (motors with interference) if the valves are open, then the breakage of the timing belt can have catastrophic effects. In fact, it is possible that before the engine stops, one or more valves (whose movements are no longer synchronized with those of the crankshaft due to the breaking of the belt) hit the pistons, damaging them. If the valve head breaks and gets stuck between the piston and the cylinder, you risk serious damage to the whole crank mechanism and / or to the cylinder block. It is therefore important to pay attention to the symptoms of a timing belt failure, which could be noises caused by the fraying of the fibers which reinforce the belt or by the detachment of rubber parts of the belt itself. If the noise is only at starting, a suspect could be the belt tensioner, no longer able to keep the belt at the correct tension. In addition to normal wear, a timing belt can break if the pulleys are damaged, the engine has overheated, the belt tensioner malfunctions or foreign bodies have entered the belt protective casing. Furthermore, the timing belt must not come into contact with the oil (there are belts that work in an oil bath but the majority are ‘dry’) because it could shorten its life, as well as having it subjected to excessive bending during assembly.


The chances of the timing belt breaking can practically be eliminated with careful fitting and compliance with the car manufacturer’s prescriptions. The company indicates, in the maintenance booklet, the replacement intervals, expressed in terms of mileage or years. An indicative distance is 120 thousand km between one replacement and the next, perhaps interspersed with visual checks every 2 years / 30,000 km. The distance indicated by the manufacturer is prudential but it is good to take into account the conditions of use: cars that move at very high speed on extra-urban roads subject the timing belt to overwork while vehicles used a lot in the city are more ‘restful’ and you could risk a few more kilometers.


‘Normal’ timing belt replacement is generally not very expensive since the distribution kit itself, which sometimes includes the water pump, can cost between 40 and 150 euros. Labor can affect much more because the operation can be cumbersome and must be done with precision because the trees must be synchronized: we are talking about figures between 400 and 1,000 euros indicatively. These costs do not vary, apart from the possible intervention of the tow truck, if the belt breaks but without causing damage. If the breakage of the belt damages the water pump or the nearby parts, the cost of the repair obviously increases and if some vital component of the engine is compromised – connecting rods, crankshaft, cylinders, distribution – the price can reach levels that do not recommend the repair. So never as in this case ‘prevention is better than cure’.


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