Timing gears keep the crankshaft and camshaft turning in proper relation to one another so that the valves open and close at the proper time.
Some engines use sprockets and chains.
The gears or sprockets, as the case may be, of the camshaft and crankshaft are keyed into position so that they cannot slip. Since they are keyed to their respective shafts, they can be replaced if they become worn or noisy. Timing the valves properly requires that the gears mesh so that the two marked teeth of one gear straddle the single marked tooth of the other. They are not built into the engine itself, but usually are attached to the engine block or cylinder head. The fuel system includes a fuel pump and carburetor mounted on the engine.