On any of the strokes of a four-stroke cycle, all the engine’s mechanical, hydraulic, and electronic components – crankshaft, connecting rods, pistons, variably timed valves, spark plugs, and more – have to be in the right place at the right time, or the engine won’t run. For the cam lobes to push valves at the correct times, the camshafts (the C in SOHC or DOHC in the engines’ specifications) are turned by heavy-duty bicycle-like chains via geared pulleys.
A larger geared pulley on the crankshaft operates those chains so that the valvetrain and intake system will turn, pull, push, and flow with exact timing.
The gravitation to chain from belt drive allows a number of efficiencies. Chains are narrower than belts, so the engine takes up less space in the engine compartment longitudinally. In addition, the sprockets can be designed to have smaller diameters than those required by belts, which cannot be bent at too sharp of an angle without compromising their strength. So engines with chain-driven camshafts can be narrower than those with belts.
Efficiencies are achieved in part by increasing the strength of the chain’s plates and joints, which in turn allows a still narrower profile. That reduces friction on the chain guides, which also helps to improve engine performance. The guides help to keep tension on the chains automatically, contributing to lower maintenance costs.