Timing chain vs. timing belt – when you need a new one

Timing belt or chain replacement is a routine maintenance that every driver must remember. When you forget about it, serious engine failure can occur. Replacement of timing is not cheap, but it is mandatory only once every few years. This way, you can enjoy trouble-free engine operation


What is engine timing?

The valve train controls the components that ensure the supply of fuel-air mixture and air to the cylinders, and also deals with the removal of combustion products (exhaust gases). It is something like the main bus that controls engine operation. The role of engine timing further increases in direct injection engines (gasoline and diesel), where the timing drives a high-pressure pump. Its job is to compress the fuel to high pressure before it is fed to the injectors. 

In addition, in some engines, the valve train also drives the water pump (which provides coolant circulation in the cooling system) and the oil pump (responsible for maintaining adequate engine oil pressure).

In a typical modern engine, the timing gear is located in the engine head. A belt or chain connects a pulley, mounted on the crankshaft, to the camshaft drive sprocket. Sometimes two camshafts (DOHC) are used, where one controls the intake and the other the exhaust valves. On the camshaft are specially shaped cams. When the shaft rotates, the opening and closing of the valves occur. In older cars, the camshaft additionally drove a distributor finger enclosed in a dome, distributing current from the ignition coil to the individual spark plugs (via high-voltage wires).

The timing belt is located on the outside of the engine (under the cover). The timing chain is built into the interior of the engine. Due to the need for oil lubrication, the timing chain hides under a metal housing.


Replacing the timing gear: what parts should be replaced?

During a routine timing belt replacement, a new belt, rollers and tensioner are installed. All components are replaced together, regardless of the degree of wear.

The replacement of the timing belt is combined with the installation of a new coolant pump. If you have bought a used car, after purchase - as a preventive measure - replace the entire timing gear. If it starts to leak between replacements, you will still pay for its replacement plus the installation of a new belt. The old belt, once removed, should not be reused

When replacing the timing chain, the following are replaced: the chain, the guide elements (sliders), and the hydraulic tensioner or tensioners. In some engines, the sprockets should also be replaced.


How often should you replace the car’s timing belt?

Car manufacturers specify exactly when the timing belt should be replaced on a particular engine model. It is replaced every certain number of kilometers of mileage, or every certain number of years, if the given mileage has not been reached in that time. 

How often should the timing belt be replaced, and why should it be replaced despite low mileage (but exceeding the service life)? You can find information on the interval in the vehicle's service book. There you will find all the key operating information. As for the age of the belt, remember that it is made of rubber - reinforced but still rubber. The material loses its properties over time, which creates the possibility of belt breakage. This is why the replacement interval is given in the number of kilometers and the maximum time of use. In most cases, mechanics recommend significantly reducing the time given by the car manufacturer—sometimes even by half. This is to prevent the timing belt from breaking.


Timing belt replacement: when is it necessary?

Examples include the timing belt becoming oily - for example, due to engine oil leaking from damaged crankshaft sealers or camshaft(s). The timing belt is also damaged by fuel, detergents and coolant. A belt soiled with the aforementioned substances can break. That's why mechanics say that the timing belt should be replaced after contact with these types of operating fluids.

Interestingly, there are cars on the market with engines that use a belt in an oil bath. These are some modern gasoline and diesel engines from the Volkswagen Group, Ford or the PSA concern (which offers cars from brands such as Citroën, Peugeot, Opel and DS – some of them you may find in the last sold vehicles list). In their case, the timing belt should be more resistant to oil. This is because it works in a so-called oil bath.


When should a timing chain be replaced?

Chain timing is, in theory, maintenance-free and should last for the entire technical life of the engine. Manufacturers recommend replacing it only if there are symptoms of failure - such as a rattling sound. It is extremely characteristic—so much so that in the case of gasoline engines, it is said to be diesel starting. Importantly, if the rattling occurs only immediately after starting, the symptoms will indicate wear of the chain tensioner. Practice, however, shows otherwise. In many modern engines, the chain does not last more than 100–120 thousand kilometers. Sometimes the timing chain also breaks without any previous symptoms.

Why does the chain get stretched? Most often, the reason is the Start/Stop system, which puts additional stress on the timing chain and its components every time the engine is turned off and started at a standstill. Moreover, there is also an oil change every 30,000 kilometers. Nowadays, most car manufacturers use such an interval, and used oil accelerates the depletion of the chain and its components. Sometimes user negligence can also be the source of problems. For example, driving with too low an oil level. 

In older, naturally aspirated engines, the chain timing unit was able to last from 300 to 500 thousand kilometers without any failures if the car owner maintained the proper oil level. In many modern drive units, such high mileage without replacing the timing chain is not possible.


What affects the price of timing chain replacement? 

In the case of replacing the timing chain, the service is more expensive than the parts because the process of implementation is more complicated than with a belt. The chain timing gear is located inside the engine, and getting to it requires the removal of many components. When replacing the timing chain, you also need to change the engine oil and oil filter, as well as the coolant.


How long does it take to replace the timing belt, and what steps does it require?

When replacing the timing belt, you need to remove the timing belt housing, tension and guide rollers, install new rollers, align the crankshaft and camshaft in the correct positions (lock them with locking devices), replace the water pump, install the new belt and tighten the tension pulley bolt.

You must always install the timing belt cover. A worn dual mass flywheel and a depleted starter affect the accelerated wear of the timing belt. This, in turn, also destroys the use of the Start/Stop system. The most important, however, is the replacement of engine oil and filters; this should be done every 10–15 thousand kilometers.


How long does it take to replace the timing chain, and what steps does it involve?

Replacing the timing chain takes a minimum of several hours. During the repair, the timing cover must be removed, the valve train locked, the chain and guide elements replaced, and the hydraulic tensioner (if damaged) replaced. The water pump and oil pump may need to be replaced, and sometimes the sprockets; the mechanic must determine their condition. However, there is one more element that determines how long it takes to replace the timing gear with a chain drive. We are talking about the location of the drive itself. Sometimes manufacturers really make life difficult for mechanics.

A perfect example concerns the BMW 3 Series E90, powered by a 2-liter diesel engine from the N47 family. Engineers used a timing chain in it. They therefore assumed that this one would become eternal. Therefore, they placed it on the side of the transmission and bulkhead between the engine compartment and passenger compartment. And this means that replacing the timing gear requires, in this case, preferably removing the engine from the vehicle. As workshop practice shows, the timing chain in this BMW engine does not last longer than 120 thousand kilometers.

The timing chain is in good condition when the proper level of engine oil is maintained. Taking care of the proper condition of the engine and avoiding overloading it due to sports driving, starting with a squeal of tires, and so on. Keeping these operating procedures in mind is important, as you won't be affected by a timing change in such a case. How often should I check the lubricant level in the engine? Remember to do this at least once a month.


Would it be possible to replace the timing gear yourself?

It is possible to replace the timing gear yourself, but it is worth considering whether it is worthwhile. A professional replacement requires keys as well as a timing lock for the car. On average, a timing lock costs from tens to hundreds of euros. A mistake during timing replacement can cause serious damage or destruction of the engine. If the timing gear is replaced in the example car every 80 thousand kilometers and the engine life is determined to be 300–350 thousand kilometers, then investing in tools for replacing the timing gear yourself and taking the risk of it makes no sense at all.

You should also keep in mind that the scale of the cost of purchasing the necessary components is only the first stage. The second, and not much less important, is knowledge. If you decide to change the belt yourself, you should know much more than how much to replace the timing gear every time. You need to have more than basic mechanical knowledge, and you should also have a workshop. Not every home garage will do the job. Lack of knowledge and lack of space can cause more complications during the replacement, which will be costly to repair, and the last mechanic will have to finish your work.

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How do I adjust the timing in the engine?

When replacing the timing belt or chain, each driven wheel must be properly aligned. Depending on the engine, this applies to the crankshaft pulley, camshaft pulley, injection pump wheel, etc. There are features, or special markings, on the wheels and on the engine housing. When replacing the timing gear, the feature on each wheel must coincide with the feature on the engine housing. Also, the belt or chain tensioner is set according to the feature—exactly in the middle. Otherwise, the engine components will not work in sync, and the drive unit will be destroyed. 

In short, the effect will be the same as in the case where the timing gear and belt simply break. There will be a collision between the pistons and the valves; the engine will stall, and it will become impossible to fire it up again.

When adjusting the timing gear, locks designed for specific makes of cars are very helpful. They allow you to lock the individual wheels in the appropriate settings and then calmly install the belt or chain without worrying about moving the wheels. 

When replacing the belt or chain, you also need to know the mileage of the belt or chain. A torque wrench with service data on the torque of individual bolts is essential. You will also need the manufacturer's manual, which describes the replacement of the timing gear step by step.

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