Timing Belts & Timing Chains
If you’ve ever bought a car, someone will no doubt have told you to “make sure you check when the timing belt was done” which is sound advice, but do you know what a timing belt actually is? and more importantly, why it’s important to know what it does and when it was last changed.
Read on and all will be explained!
What are timing chains and timing belts?
Timing chains and timing belts perform the same function, they connect the camshaft/s and the crankshaft to keep them rotating in-sync. Despite having the same functionality they are unique in their design and each of them have their own benefits as well as disadvantages. As they have the same function, an engine will, generally speaking be fitted with one or the other (there are a handful of engines that have both). They are generally not visible when you open the bonnet as they are are protected by metal or plastic covers to guard them from debris.
In very simple terms, an engine is essentially split into two halves, the top half (comprising of the cylinder head, camshafts and valves) and the bottom half (comprising of the cylinder block, crankshaft and pistons) The top and bottom halves of the engine must rotate precisely in time with each other and it is the responsibility of the timing belt or chain to ensure this happens. If the timing goes out of sync by even a couple of degrees (due to a worn timing belt/chain or one that was set up incorrectly in the first place) the engine will not run smoothly and is likely to show some of the following symptoms:
- Miss-firing or backfiring through the intake or exhaust
- Rough running
- Difficulty starting /stalling
- Lack of power
Timing belts are made of rubber which is reinforced with steel wire. As the belts are flexible and there is no metal on metal contact it eliminates the need for lubrication. The majority of manufacturers recommend changing the timing belt between 60,000 and 100,000 miles, although it’s important to check with your manufacturer to find out the exact recommendation. Since timing belts are fed through a series of pulleys and tensioners it’s important that these are replaced at the same time as the belt. It is also a good idea to change the water pump at the same time since it has done as much work as the belt. In newer cars the water pump is often installed behind the belt so the belt has to be removed to change it. In other words, you may as well change the water pump since the belt is being changed.
Timing chains are made of metal and resemble the chains that are on bicycles and motorbikes. As there is metal on metal contact the chain runs through the inside the engine so it can be lubricated by engine oil. As a general rule, timing chains only need to be replaced if there is a problem with it. Their main benefit is that they last for a long time. Some disadvantages of timing chains include the louder noise level and the damaging impact from one if it breaks.
Similar to timing belts, they are fed through a series of tensioners and pulleys to keep them under the optimum level of tension. Timing chain tensioners rely on engine oil pressure, if for any reason the oil pressure is low the chain tensioners can lose pressure and it will offset the engine timing. This results in poor engine performance and the chain may malfunction causing expensive damage. Timing chains usually have no connection to the water pump unlike timing belts so they don’t need to be replaced at the same time.
Symptoms of a worn timing chain:
- One of the most common symptoms of worn timing chains is noise. Worn timing chains can become loose and it can hit off the protective covers as it rotates. It can also result in some debris around the engine as it hits off the cover. A rattling timing chain is usually on the verge of breaking point and it should be replaced immediately.
- A rough idling sound can indicate a worn chain, as the engine’s valves are running out of sync. This will be most noticeable when you start the car as it tends to shudder or shake.
- If the chain is very worn it may no longer be positioned correctly on the gears hindering the engine from starting.
- Backfiring, shaking or shuddering while driving.
Preventing timing chain problems
- Use the correct specification of engine oil for your vehicle
- Change oil and oil filters on the recommended intervals
- Have any unusual sounds in the engine checked
What happens if the timing belt or chain snaps?
When talking about timing chains and belts it’s important to know about interference engines. An interference engine is one where the pistons and valves share space in the cylinder but not at the same time (the vast majority of engines on the road today are interference engines). The benefit of this type of engine is efficiency, however if the belt or chain breaks in an interference engine the valves and pistons will collide. The pistons generally hit the valves which will bend or snap them or else the valves will punch a hole in the top of the pistons. The repair bill could easily run into the thousands. If the belt breaks in a non-interference engine they won’t collide as they do not ever share the same space.
So knowing when the timing belt was last changed is a very good thing!