Cams are forged out as an integral part of the shaft. The lift of a cam is ordinarily 5/16 in., sometimes running as high as 3/8 in. The lift of the cam is usually the lift of the valve, or the amount of opening for the valve. When rocker arms are used the pivot point of the arm has an influence on the total valve lift.
The length of time a valve may be held open is determined largely by the width (dwell) of the cam nose. This is termed "dwell" because it holds the valve open. The valve lifters, whether they are of the mushroom type, or the hydraulic type, ride directly upon the cams. Designers give much thought and attention to the task of getting the valve lifter to follow over the cam in a proper fashion. It is difficult to obtain quick valve lifting and return and at the same time produce a valve action that will remain quiet, positive, and smooth-running for a long time. Strong valve springs are used to cause the valve lifter to follow the contour of the cam.
Camshafts are designed with the journals a bit larger than the cams are high. This is necessary to permit removing the camshaft endwise from its bearings in the crankcase.
Timing gears are the oldest form of camshaft drive. Many materials have been used for timing gears and timing-gear idlers, with the idea of silencing them. Paper, fibre, and similar composition gears, and aluminium, bronze, and similar metals have been used in attempts to get away from disagreeable timing-gear noises. Timing-gear noises are due to the irregular load placed on them from the action of the valve lifters and valve springs. This is especially noticeable at low speeds, causing them, when badly worn, to knock and rattle.
Silent-chain or link-belt drive is largely used for camshaft and accessory drive. It is free from disagreeable noises. The chief objection is looseness, due to stretching. Stretching is due to wear within the chain links. An idler gear, which may be adjusted automatically or from an eccentric arrangement controlled from the outside, may be provided to compensate for wear. The automatic take-up is controlled by spring action, and the chain runs under proper tension at all times. Most chains have arrows stamped on them indicating the direction of drive. The eccentric-bushing and idler-gear adjustment requires manual adjustment from time to time.
Timing marks are placed on chains and sprockets so that the engine (valve) timing may be set, on occasion. They are not so easily found as in the case of timing gears, but may he detected on the chain and sprockets.