Camshafts
 

One of the most important components in any engine is the camshaft. Whether the camshaft is in a pushrod 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, fuel economy 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 crankshaft 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 four stroke 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 of reasons. 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 cam-driven. Most OHC engines that use a chain drive 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-driven 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 freewheel.
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 interval. On newer OHC engines, it is 100,000 miles.
Cam failures can occur if there are lubrication problems in the engine. Lifters create a lot of pressure and friction on the cam lobes, so the lobes and cam bearings must receive lots of oil. If oil pressure is low or the oil is dirty, the cam may suffer accelerated lobe wear and ultimately lobe failure resulting in a dead cylinder (no valve action). This type of cam damage can also be caused by using the wrong viscosity motor oil. In overhead cam engines, it is a long way from the oil pump to the top of the cylinder head. On cold mornings when the oil is thick, it can take quite a few seconds for adequate oil pressure to reach the cam. That is why most vehicle manufacturers recommend using 5W-30 oil rather than 10W-30 or 10W-40 for cold weather driving.
Cam breakage or seizure is another problem that can occur in OHC engines. The cause may be inadequate lubrication but in many instances it is caused by head warpage. When an OHC engine gets too hot, the cylinder head tends to swell and bulge up in the middle. This changes the alignment of the cam bores in the head which may cause the cam to bend, bind, seize or break. If an overhead cam won�t turn freely in the head when the belt and cam followers are removed, either the cam is bent or the head is warped and needs to be straightened and/or align bored.