- Timing belt
- Timing belt with separate camshaft belt
- Distribution chain
- Assembly overview of a timing chain of a Volkswagen engine
- Assembly overview of a timing chain of a BMW motorcycle
- Combination of belt and chain
The distribution is mainly responsible for the drive of the camshaft, possibly the water pump and-/or fuel pump with a diesel engine. The water pump can be driven by the timing belt, but also by the multi-belt. Today, many manufacturers even use electronically powered water pumps. The distribution can be driven in several ways. These are described further down the page.
It is important that the distribution is on time. When disassembling/reassembling the belt or chain it is necessary to zero the marks on both the crankshaft, camshaft and in some cases the fuel pump in diesel engines (set to time). This is indicated by markings on both the engine block and the gears or pulleys, or is the installation of blocking tools the only way to set the timing on time. Securing the sprockets with special locking pins prevents them from turning unintentionally when replacing the timing belt or chain.
After installing the belt, the tension should also be checked and adjusted. This can be done by means of a tensioning roller with a hydraulic tensioning element or with manual adjustment option. With a timing chain, the tension is automatically regulated by a hydraulic chain tensioner, which tensions more or less depending on the oil pressure.
The image makes clear what is meant by timing. Above are the two camshaft sprockets. There are (red) markings on these two gears. In reality, these are often two notches that have been ground into the sprocket. According to the instructions, they should face each other (left gear at 3 o'clock and right gear at 9 o'clock).
The crankshaft gear also has a marking. Here too, a notch is often cut. The marking point should match the marking point on the engine block. When the marking points on the crankshaft and the engine block are opposite each other, but the points of the camshafts are rotated 180° (i.e. half a turn), the crankshaft must be turned one more revolution. Each camshaft gear has twice as many teeth, so when the crankshaft rotates 2 rotations, the camshafts are rotated 1 rotation.
If the distribution is not on time, the valves will open too early and close too early (or open too late and then close again late). This can lead to poor starting, poor engine running and loss of power. In the worst case where the timing belt is out of position one or more teeth relative to zero, the pistons can hit the valves. In most cases, this leads to a lot of engine damage. With some engines, engine damage can already occur if the timing belt is only one tooth wrong. So pay attention to this during installation.
A timing belt lasts (depending on the type of engine) about 60.000, 90.000, 120.000, 180.000 or sometimes 250.000 km. In addition to the mileage, there is also a certain time, eg a maximum of 4 years. As the belt ages, it can dry out or in the worst case tear, causing the belt to break over time. The advantages why a manufacturer chooses a timing belt are:
- Does not need to be lubricated as is the case with a chain and sprockets (today, timing belts are used in combination with motor oil, eg by Ford).
- Low weight
The disadvantages of a belt are that it needs to be replaced periodically and that it is sensitive to grease or coolant, which can escape from the crankshaft and camshaft seals or the water pump, if it is mounted in the distribution section.
Timing belt with separate camshaft belt:
The picture below is of a twin cam engine driven by two timing belts. The camshaft gear (1) is directly driven by the crankshaft with the large belt. At the back of the pulley of gear 1 is a small gear, over which the rear belt runs. This rear (small) belt drives the camshaft gear (2). The small belt needs a separate tensioner. Usually this is applied to four-cylinder engines with 16 or more valves. (so 4 or more valves per cylinder)
A timing chain is virtually maintenance-free, unlike a timing belt that has a replacement period. A number of manufacturers have switched back to the chain after improvements in noise and friction. A timing chain can last a lifetime. The tensioners that are used here often work on hydraulic pressure and spring force, so that the tension of the chain does not have to be checked regularly.
The image below shows an engine with a single camshaft fitted with a timing chain. The crankshaft sprocket drives the camshaft sprocket by means of the timing chain. The chain slides along the plastic chain guide. This is, of course, lubricated by motor oil. The hydraulic chain tensioner presses the plastic chain guide against the timing chain. The guide pivots around the hinge point at the bottom. This will press the chain tight. There must be no play on the chain, otherwise it may chatter. The hydraulic chain tensioner contains a strong spring, which ensures that the chain remains tensioned even when the engine is switched off. When the engine is running, the oil pressure in the chain tensioner will help push the chain tensioner against the guide bar with more force.
The oil pump gear is driven by a separate oil pump chain. The operation of the oil pump with separate chain is described on the page lubrication system.
It can happen that a timing chain becomes longer. Then there is a stretched chain. Every chain gets a little longer as it gets older, but that is often within tolerances. However, it can happen that a chain stretches so far that malfunctions can occur. Possible causes of a stretched chain include:
- The type of material the chain is made of affects its lifespan.
- Driving at too low revs causing additional engine vibrations.
- Very sporty driving style with a lot of acceleration.
- Outdated engine oil due to insufficient changes.
The following symptoms indicate a stretched timing chain:
- Irregular running of the engine due to incorrect valve timing (the valves no longer open and close at the correct time)
- The Engine Malfunction Light (MIL) may illuminate. An error code may then be present that relates to incorrect timing between crankshaft and camshaft.
- Rattling noises after starting and starting the engine. This may be because there is play between the chain and guide bar and the chain tensioner still needs to be filled with oil before it can remove the play. After the chain tensioner has further tightened the chain, the rattling noise may disappear, but this can become worse over time as the tensioner has to move further out.
- Rattling noises while the engine is running. If a constant rattling noise is heard from the timing chain, it could be that the tensioner has moved out to its maximum and there is still play between the chain and the guide bar. This is a sign that you should definitely not drive any further, because there is a good chance that the chain can slide over the sprockets. As a result, the pistons can hit the valves, causing serious engine damage.
As the chain rattles, it hits the plastic guides. Especially with older conductors, where the plastic has become hard, there is a good chance that pieces of plastic will break off. Besides the fact that the chain now has even more space on the guide, the broken piece of plastic can end up in the oil pan. As a result, the piece of plastic gets stuck in front of the suction strainer and blocks the flow of oil to the oil pump. This results in an oil pressure that is too low and therefore lubrication problems for the engine parts. Ultimately, this can cause fatal engine damage.
Multiple timing chains:
Several timing chains can also be mounted in an engine. There are four separate chains in the V-motor in the picture. Chain 1 drives the oil pump and the balance shafts. Chain 2 is connected to the crankshaft and the camshaft drive sprockets. Chain 2 and chain 3 and 4 are driven by which the camshaft sprockets are driven. Each chain is tensioned by its own chain tensioner.
Assembly overview of a timing chain of a Volkswagen engine:
The image below shows a Volkswagen engine with a cutaway distribution housing. The upper and lower timing covers have been removed. These are mounted with kit and about 25 bolts. In the lower timing cover is also the crankshaft seal to see. The lower timing cover is also located on the underside of the oil pan mounted.
When the timing chain needs to be replaced, the crankshaft must be blocked by screwing a special bolt into the engine block. The crankshaft must be turned until the crankpin touches the bolt. The crankshaft is then in the correct position.
The camshaft should be blocked by sliding a special tool with a protruding part into a slot at the rear.
Assembly overview of a timing chain of a BMW engine:
The image shows a distribution cassette of a BMW engine. This BMW motorcycle has no covers that need to be disassembled to replace the chain. The timing cassette must be lifted completely out of the engine. How this works is briefly described below. A number of disassembly and assembly steps are not shown here because it is not a repair manual, but is only intended to give an impression of the operation and assembly. So always consult the repair brochure where the tightening torques are also described!
The timing cassette shown consists of the timing chain, a plastic timing guide, a crankshaft sprocket and two (adjustable) camshaft sprockets. When mounting, the conductor must first be assembled as shown below.
When the bolts of the guides, the crankshaft sprocket and the camshaft sprockets are removed with the chain tensioner and valve cover removed, the entire cassette can be lifted out in its entirety.
Before tightening the camshaft sprockets, the impulse discs should be put in the correct position by fitting a tool. The protruding points in the tool are pushed into the holes of the impulse discs so that they are in the correct position.
Because the crankshaft sprocket and the sprocket for the balancer shafts are behind each other, the balancer shafts must first be fixed before the crankshaft bolt is tightened. If this is not done and the balance shafts are not on time, it will actually amplify the engine vibrations instead of damping them.
After disassembling the crankshaft bolt, the balance shafts drop to their lowest point. The chance that the balance shafts will twist is greater than that they will remain perfectly aligned.
When crankshaft, camshaft and the balance shafts are fixed, the crankshaft flange can be slid in. The bolt of the crankshaft can be screwed in a few turns, but does not have to be tightened yet.
Before the crankshaft bolt is tightened to the correct torque, the chain guide must first be pressed against the timing chain with a certain force in order to obtain a certain preload. This can be done with the special tool that must be mounted in the hole of the chain tensioner. By tightening it with a prescribed torque, the timing chain is sufficiently tensioned. Now the crankshaft bolt can be tightened.
Always use a special tool to tighten the crankshaft bolt.
When the bolts of the camshafts, guides and the crankshaft are tight, the hydraulic chain tensioner can be installed.
After tightening and removing the special tool, the crankshaft must be turned two rotations clockwise (clockwise). The timing should be rechecked. If it is not correct, the adjustment procedure must be repeated.
Combination of belt and chain:
The picture shows a twin cam engine driven by both a belt and a chain. Camshaft 1 is driven by the timing belt (left). The crankshaft drives this belt. The drive for camshaft 2 is provided by means of the timing chain.
Some engines only use gears in the distribution. The big advantage is that this is completely maintenance-free and there is no chance that the belt will age, the belt or chain will not be properly tensioned, etc. The big disadvantage is that it produces a lot of noise. The helical gearing on the sprockets already dampens a lot of noise, but it still makes a lot more noise than a belt or chain. That is also a big reason why this type of distribution is not used very often.