Many of today's cars are equipped with a timing belt in place of the old timing chain. The function of this small yet critical member is to keep your engine mechanically "in time." How they work: In a four-stroke internal combustion engine (intake, compression, power, exhaust) the top half of the engine must be synchronized (or in time) with the bottom half to complete the four stroke cycle. The timing belt achieves this by meshing with cogs connected to the crankshaft and camshaft. The driving of these components in perfect time achieves the four stroke cycle, producing power in the engine. The timing belt is made out of rubber and is subject to wear and tear due to mechanical and environmental conditions. The bottom line with timing belts is to have them checked every 25,000 miles and replaced every 50,000 miles. Telltale signs of a failing belt are cracks, cuts, worn or broken teeth, and deterioration from wear and exposure to harmful fluids and/or high temperatures. If the belt's teeth are gone, the valve timing can be affected by either excessive advancement or retardation, resulting in poor engine performance. If the belt breaks, major engine damage can occur on some engines.