An Interference engine is a type of internal combustion engine where in the event of a serious valve train failure, or a 180 degree timing error, could lead the valves and pistons to strike each other. The most common cause of this a broken timing belt, but a catastrophic timing chain failure can also bring about this situation.

The reason for this is that for an engine to work properly, the actions of the pistons and valves must be precisely harmonized. This is accomplished in three ways. In racing engines, gears may connect the camshaft and crankshaft. In street engines, that task is accomplished by a timing chain or belt. When the chain or belt is installed, the camshaft and crank must be turned so that the number one cylinder is at Top Dead Center, or when it is at the maximum of its stroke. Generally, there are timing marks on the timing gears. or pulleys to aid in this, and the gears themselves are usually keyed to prevent misalignment. However, some people do screw it up.

If the belt or chain breaks, then this relationship is destroyed, and the valves and pistons move independently. In most automobile engines, this does no serious damage. The motor simply stops running until the belt/chain is replaced and the engine retimed. Even when the valves and pistons are at full extension, a small gap remains between them. We call this a non-interference engine. But such designs are not ubiquitous, and designers seeking a small combustion chamber may produce an interference design. The Porsche 944 3.0, and Volkswagon/Audi 1.8T engines are interference engines.

Why does this matter?

If the chain or belt breaks in an interference engine, the piston and valve may strike each other. Actually, they will almost certainly strike somewhere. The least this will do is bend the valve. The piston may also be damaged, and bits of metal may work loose in the engine. An engine rebuild is required, with engine replacement a possibility. With an older, inexpensive car sometimes the only rational solution is to throw away the car.

How to Avoid a Valve Piston Moment

Timing Chain failures are very rare. A timing chain is basically a short bicycle chain, often with doubled links. They are usually good for well over 100,000 miles. When they fail, it usually involves stretching. If your mechanic tells you he's having a hard time timing an older car, that is a sign of a stretching or loose chain (which may indicate a tensioner problem.. Timing chains generally give you a LOT of warning before they go. You may get a year or more.

Timing belts are different. They are really nothing more than a wide version of the serpentine belt that operates your car's accessory drives, such as the power steering pump and alternator. They are cogged, to permit precise timing. Timing belts break with little or no warning. While belt life varies, most require replacement every 60,000 miles. 30,000 miles is recommended for Porsche owners. Check your vehicle's service manual. It will tell you whether you have a chain or belt, and if a belt, its expected life. The manual will also tell you whether or not your engine is an interference design.

Haven't got a service manual? Buy one. If you plan to keep the car, they are very useful things to have even if you don't do your own work. Like what to expect when a repair bill is presented. Service intervals. At about $25 for a Chiltons, more for a factory manual, they are one of the wisest investments you can make.

Do not assume that your mechanic or dealer will automatically change the belt when you bring it in for recommended service. People forget. This is a maintenance interval you need to know, and you need to remind them. A woman I know just spent $5K because her Audi dealer forgot the belt change.

If you don't have an interference engine, you can relax a bit. A belt failure will only leave you stranded. You get to call a friend and a tow truck and then pay to have the belt replaced. Which if you had done already, you wouldn't be walking.

But if you do have an interference engine, you need to take steps right now. If the car is an older model, go through your service reciepts to see if, and when, the belt was changed. If it was 30K miles ago, you can relax, you probably have two years left before you need worry. But if you car has passed the mileage window for a change, and you don't know if the belt has been changed, and you plan, change the belt.


Belt replacement is considered routine maintenance. It's fairly simple, and starts at about $100, though it may cost more depending on what the mechanic has to remove to get to the belt. Timing chains are harder to change, which costs more money, but last a lot longer. If you change either belt or chain, the tensioner must also be changed. If you have to change a timing chain, the timing gears should also be changed. When your mechanic tells you he wants to change the gears, he's not ripping you off. The gears cost about $20 each, the labor to replace them in the hundreds. It's not worth the risk to leave them. A good principle in engine work is that if you've opened up the motor, do everything while you're there. It will save you money and trouble in the long run.

Once you've changed your timing belt, you can reset it's life clock to zero, meaning you should have years of driving without a worry.

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