Figure 2.2 shows an OHV drive in which the valves are driven by the camshaft via cam followers, push rods, and rocker arms. Since the drive to the camshaft is simple (either belt or chain) and the only machining is in the cylinder block, this is a cost-effective arrangement.In the OHC drive shown in Fig. 2.3 the camshaft is mounted directly over the valve stems. Alternatively it could be offset and the valves operated using rockers. The valve clearance could then be adjusted by altering the pivot height. Once again, the drive to the camshaft is by toothed belt or chain.
In the system shown in Fig. 2.3 the camshaft operates on a follower or bucket. The clearance between the valve tip and the follower is adjusted by a shim. This is more difficult to adjust than in systems using rockers, but is less likely to change. The spring retainer is attached to the valve using a tapered split collet. The valve guides are usually press-fitted into the cylinder head, so that they can be replaced when worn. Valve seat
inserts are used to ensure minimal wear. The valves rotate in order to ensure even wear and to maintain good seating. This rotation is promoted by having the cam offset from the valve stem axis. This also helps to avoid localized wear on the cam follower
E N G I N E E R I N G R E S E A R C H S E R I E S
Automotive Engine Valve Recession
R Lewis and R S Dwyer-Joyce
Series Editor
Duncan Dowson
Professional Engineering Publishing Limited,
London and Bury St Edmunds, UK
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