I used to have an 1987 Honda Civic 4-door sedan (one of the big ones, like an Accord). This car was virtually indestructible and seldom needed repairs, even after 130,000 miles. Then one day the engine just stopped as I was driving down the highway, and would not turn over. My timing chain broke. The resulting damage (hundreds) was a lot more expensive than if I had just had the timing chain replaced per the manufacturer’s schedule ($150 at 100,000). Don’t let this happen to you.
The timing chain connects the crankshaft to the cam shaft. The crankshaft is the part turned by the pistons in an internal combustion engine, and delivers the power to the wheels. The cylinders in which combustion takes place (and pushes down the pistons) have valves which let in air and fuel and then close before the fuel/air mixture is ignited. A cam shaft is a metal rod with oblong metal blobs or “cams” which push on the valve lifters. The timing chain thus links the opening and closing of the valves to the speed of the rotating crankshaft.
Some cars have timing chains, others have a timing belt: generally, overhead cam (OHC) engines have timing belts (the distance between crankshaft and cam shaft requires use of a quieter and lighter belt) while overhead valve (OHV) design engines have timing chains. Chains do not have to be replaced as often as belts, but are heavier, noisier and more expensive than belts. Timing chains used to outlast vehicles, but these days, with outstanding vehicles like the Honda, even the chains have to replaced once or twice in the life of the car.
Either way, a broken timing belt or chain can be catastrophic if you have an “interference fit” type engine, meaning that the pistons will hit the valves when the chain breaks. If you are driving, your engine will stop instantly, but the crankshaft will keep turning as your car coasts to a stop. Since the mechanism that lifts the valves out of the way is no longer working, in the “intereference fit” engine the pistons will hit the valves, bending them and damaging the whole assembly. This is very bad news.
Since timing chains sometimes don't break until the car has over 100,000 miles on it, the repair cost is likely to be more than the replacement value of the car.
Most cars sold in the past 10-20 years are "intererence fit" and will be damaged severely if the timing chain or belt is allowed to break. Incomplete list of cars with "interference fit" engines that I'm certain will bend valves if the timing belt breaks: Nissans, Mitsubishis, Hondas with 16-valve, four cylinder engines (like my old Civic); Chrysler compacts with Mitsubishi engines (Lazer, Neon), Fiat Spyders. If you own one of these, get the timing belt/chain replaced when it's scheduled or you’ll be sorry!
Timing Belt Replacement Charts: www.theautoshop.com/timing.html
(Also indicates which engines are “interference fit” and therefore would be damaged if belt breaks).