Most modern cars that have a timing belt are what are called interference engines. This means that the intake and exhaust valves move into the area where the piston moves, but at different times. The timing belt (or chain) makes sure that the valves open and close at the proper time to make the engine function correctly. My last blog about how an engine works explains how the valves and pistons function. When a timing belt breaks the valves stop moving. Some of them will stop moving in a position that will be in the path of the still moving pistons. Your engine spins at thousands of revolutions per minute, and therefore the piston is moving with a lot of force toward the valves. Any valves in the pistons path will get bent by the collision. Any bent valves will need to be replaced. This is a major undertaking, and will be very expensive. You will go from hundreds of dollars to replace the belt to thousands of dollars to repair the engine. Timing belts are made of rubber and simply wear out over the course of time. It will break without any warning, your car will stall, and you will need a tow truck. Sometimes customers will ask us to check the timing belt. The only way to check a timing belt with any level of accuracy is to remove it and look at it closely. In most cases it takes several hours to remove and replace a timing belt, so if you take the belt off you might as well put on a new one. Because it is not practical to check the belt the manufacturers pick a mileage interval, depending on make and model, somewhere between 60,000 miles and 110,000 miles. In most cases it is wise to replace the timing belt tensioner. Its job is to take up the slack in the belt as it stretches over the years. If the tensioner fails the same collision of valves and pistons occurs, so even with a new timing belt you could still end up with an expensive problem. Often the timing belt drives the water pump which pumps the coolant throughout the engine. You will save a lot of money on labor to replace the pump at the same time as the belt. The water pump can also cause extensive damage if it fails, so it makes financial sense to replace it preventatively at the timing belt interval. Some engines have a timing chain and do not need to be replaced for maintenance. Timing chains as a general rule only fail if proper oil change intervals are not adhered to. Replacing a timing chain can be very expensive, and oil changes are your best preventative measure for not only the chain, but other expensive parts inside your engine.