timing gears of a four-stroke engine Instead of toothed belts or a chain, 10 spur gears are used to drive the injection pump, the camshaft and the ancillaries. Injection pump and camshaft turn with half the crankshaft speed. description ordernumber 479 see also: valve timing Four-stroke engine with one overhead camshaft (OHC) valve-timing diagram two-stroke engine with rotary-disk valve control