Here’s a question we get asked all the time. “What does a timing belt do and why does it cost so much if it fails?”

Most people have had the unfortunate experience of a timing belt breaking on their vehicle. When it does it usually costs a tow and causes expensive internal engine damage as well as loss of their vehicle for several days.

But why does this happen? Well to illustrate I have included some images. This first image is from a four cylinder engine with 2 camshafts commonly known as a dual overhead cam engine. I have labeled some of the parts such as the crankshaft and cam shafts. This particular engine has a water pump also driven by the timing belt. There is a tensioner pulley and 2 idler pulleys that guide the belt. A failure of the water pump or any of the guide pulleys has the same result as a failed belt.

front of an engine showing timing belt, camshafts, crankshaft and water pump

And here is why! I have included an engine cutaway animation depicting what happens inside an engine while it is running. The timing belt is not actually shown in this image. The crankshaft is the large shaft at the bottom of the engine which is connected to a piston moving up and down. The cam shafts are depicted by the 2 bright green small oblong shafts at the top of the engine. The cam shafts are responsible for opening the intake and exhaust valves. Note that there are locations where the valves occupy the same space as the piston just not at the same time (btw: that’s why it’s called a timing belt). The crankshaft is what delivers the power from the engine to your vehicles wheels via the drivetrain. The crank shaft can rotate at speeds of 500rpm to well over 6000rpm on some engines. While driving, engine speeds of 1500rpm to 2500rpm are fairly common. At 2000rpm the piston would go down and back up just over 33 times per second. Should the timing belt fail the camshafts will stop turning (very rapidly) and some of the valves will be stuck in an open position occupying space where a piston is going to be. The crankshaft will NOT stop right away especially while connected to your vehicle’s wheels. The pistons will keep going up and down and they are going to lay a good smack down on those open valves and that is the expensive part.


Most timing belts call for around a 60,000 mile (100,000km) change interval and water pumps commonly fail around 80,000 miles (130,000km). Since most of the labor to replace a water pump is the same as doing a timing belt do you feel it would be wise to replace a water pump when doing a timing belt? What about the timing belt guide pulleys? There are companies that provide timing kits consisting of the timing belt, guide pulleys and water pump usually at reduced prices compared to purchasing the pieces separately.