<P> The overhead camshaft design adds more valvetrain components that ultimately result in more complexity and higher manufacturing costs, but this is easily offset by many advantages over the older design: multi-valve design, higher RPM limit, and design freedom to better place valves, spark plugs (Spark - ignition engine), and intake / exhaust ports . </P> <P> The rockers or cam followers sometimes incorporate a mechanism to adjust the valve lash through manual adjustment, but most modern auto engines have hydraulic lifters, eliminating the need to adjust the valve lash at regular intervals as the valvetrain wears, in particular the valves and valve seats in the combustion chamber . </P> <P> Sliding friction between the surface of the cam and the cam follower which rides upon it can be considerable . In order to reduce wear at this point, the cam and follower are both surface hardened, and modern lubricant motor oils contain additives specifically to reduce sliding friction . The lobes of the camshaft are usually slightly tapered and the faces of the valve lifters slightly domed, causing the lifters to rotate to distribute wear on the parts . The surfaces of the cam and follower are designed to "wear in" together, and therefore each follower should stay with its original cam lobe and never be moved to a different lobe . You can put new lifters on an old cam but never old lifters on a new cam . In some engines the followers have rollers which eliminate the sliding friction and wear but add mass to the valvetrain . </P> <P> Camshaft bearings are similar to crankshaft main bearings, being pressure - fed with oil . However, overhead camshaft bearings do not always have replaceable bearing shells, meaning that a new cylinder head is required if the bearings suffer wear due to insufficient or dirty oil . </P>

What does a cam do in an engine