One of the most important components in any belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine is the camshaft. Whether the camshaft is in a belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine or an overhead cam engine, it controls the opening and closing of the valves. This, in turn, controls the flow of air and fuel into and out of the engine which determines engine performance, belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine and emissions.
In pushrod engines, the camshaft is either chain or gear driven off the crankshaft. In OHC engines, the camshaft may be belt or chain driven from the belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine or an intermediate shaft. The drive ratio is always 1:2 so the cam turns at half the speed of the crankshaft. This is because the crankshaft in a belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine engine makes two complete revolutions for every power cycle (intake stroke, compression stroke, power stroke and exhaust stroke).
Cam-related problems can occur for a variety belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine. As an engine accumulates miles, the timing chain stretches. The added slack in the chain has a retarding effect on cam timing, which reduces compression and torque. It can also retard ignition timing if the distributor is belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine. Most OHC engines that use a belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine have some type of automatic chain tensioning device, but pushrod engines do not. Consequently, the timing chain and gear set often need to be replaced in high-mileage pushrod engines.
In OHC engines with belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine cams, the main concern is belt failure. If the belt snaps, the cam stops turning and the engine quits. Some valves will be held in the open position, which may result in bent valves and/or damaged pistons if the engine does not have enough clearance between the pistons and valves to belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine.
To minimize the risk of such damage, most vehicle manufacturers recommend replacing OHC timing belts at specific mileage intervals for preventive maintenance. On older OHC engines, 60,000 miles is the typical replacement belt-drivencam-drivenchain drivecrankshaftenginefour strokefreewheelfuel economyintervalof reasonspushrod engine.