valvetrain

The valvetrain is the group of engine components that controls the opening and closing of the intake and exhaust valves. It translates the rotary motion of the camshaft into the linear movement needed to operate the valves in time with the pistons, thereby regulating air-fuel admission and exhaust scavenging to determine engine power, efficiency, and emissions.

Key components include the camshaft(s), lifters or tappets, pushrods (in pushrod or OHV layouts), rocker arms, valves, springs, retainers, seals, and guides. The timing mechanism—which may be a timing belt, chain, or gears along with tensioners and guides—keeps the camshaft(s) synchronized with the crankshaft. Valvetrain configurations differ: overhead cam (OHC) engines place the camshaft in the cylinder head, often with direct or rocker-arm-assisted valve actuation, while overhead valve (OHV) or pushrod engines place the camshaft in the engine block and use pushrods to operate the rocker arms.

Lifters can be hydraulic or solid (mechanical). Hydraulic lifters automatically adjust lash for quiet operation, while solid lifters require periodic adjustment for precise timing. Valves include intake and exhaust types, each seated in hardened valve seats with springs and retainers to close them under engine velocity. Proper lubrication minimizes wear on cam lobes, lifters, pivots, and valve stems.

Maintenance considerations include regular oil changes, inspection for wear or noisiness, and replacement of timing components as needed. Performance-oriented valvetrains may employ variable valve timing (VVT), variable lift or duration systems, and higher-lift cams to improve breathing at high RPMs. Overall, the valvetrain is central to engine breathing, efficiency, and responsiveness.