The camshaft can be cast or machined. It drives the valves and the distributor. It has a cam for each valve. Some also have a cam known as an eccentric to drive the fuel pump.
The overhead camshaft is located in the cylinder head. Double overhead cam systems often use a bucket tappet. Some overhead cam engines use a hydraulic lash adjuster to eliminate need for valve clearance adjustment.
The cam lobe opens a valve at the proper time and gives it proper lift. It lets it stay open for a sufficient time, then lets it close at the proper time. Accurate valve timing is crucial.
Timing belts and chains are used on overhead camshaft engines, because the camshaft is further from the crankshaft. Timing belts are quieter than chains but usually require regular tensioning and replacement.
