Pushrods (or rods)

Pushrods are slender metal rods whose job is to carry the motion of the camshaft up to the valves in a particular and long-established style of engine. They exist because of where the camshaft sits. In an overhead-valve engine the valves are placed in the cylinder head, at the top of the engine, but the camshaft that decides when those valves open is mounted low down within the engine block, close to the crankshaft that drives it. Something must bridge the considerable distance between the two, and the pushrod is that bridge.

The operation forms a chain of components working in sequence. As the camshaft rotates, each lobe lifts a follower, sometimes called a tappet or lifter, riding on its surface. The follower pushes the lower end of the pushrod upward. The rod, running up through a passage in the block and head, transmits that lift to a rocker arm pivoting at the top of the engine. The rocker arm then bears down on the valve stem, opening the valve against its spring; when the cam lobe rotates away, the spring closes the valve and the whole train follows the motion back down. This is the overhead-valve, or OHV, layout, distinct from designs that place the camshaft up in the head.

The enduring appeal of the pushrod arrangement lies in its compactness and simplicity. Because only the valves and rockers live in the cylinder head while the single camshaft stays in the block, the head can be small and the overall engine remarkably narrow and short for its capacity. This is why the layout has dominated the American V8, where a large-displacement engine can be packaged neatly between the front wheels. The single low camshaft, driven by a short, robust chain or gears, also keeps the valvetrain inexpensive and mechanically uncomplicated compared with multi-camshaft alternatives.

The trade-off appears at high engine speeds. The pushrod valvetrain carries more moving mass and more links in its chain than an overhead-camshaft design, and at high revs this mass is prone to flexing, bouncing and valve float, where the valves no longer follow the cam profile faithfully. Overhead-cam engines, which act on the valves far more directly, generally rev higher and breathe more freely, which is one reason most high-revving and four-valve-per-cylinder designs abandoned pushrods. Engineers have nonetheless kept the layout competitive through lightweight components, hydraulic lifters and careful design.

In the wider scheme of valvetrains, pushrods sit at one end of a spectrum running through single overhead camshaft and double overhead camshaft arrangements. They remain prized where a wide, torquey power delivery, compact dimensions and low cost matter more than ultimate engine speed. Far from being merely a relic, the pushrod engine survives in large performance V8s precisely because its blend of simplicity, packaging and low-end muscle still suits certain applications very well.