(d) WORN CAM-SHAFT BEARINGS are the cause of trouble, as they will change both the timing and the lift of the valves. If much play exists in the bearing, it will prevent the valves from lifting at the proper time, and will also reduce the lift by the amount of the play, which sometimes has a considerable effect on the free passage of the gases. If the cam-shaft bearings are of the bushing type they should be replaced with new paying attention at the same time to the condition of the shaft. If rough or shouldered the shaft should be machined to a dead smooth surface. If on a large engine and of the adjustable type, the shims should be removed as required or the wedges adjusted.
(e) LOOSE CAMS OR ECCENTRICS will change the timing because of lost or sheared keys. If your cams are not integral with the shaft, look them over occasionally and be sure that the keys are tight. Loose cams will produce thumping and grinding and may often be located by the sound. See that the key-way is not worn when fitting keys.
If the cams are fitted with taper pins it would be well to ream the hole before placing new pins, as there is a liability of the hole being worn oval.
(f) A TWISTED OR SPRUNG CAM-SHAFT will change the positions of the cams relative to one another, and not only will change the time of all cylinders, but will change their time relatively causing the engine to run out of balance, or produce an unusual vibration.
(g) WORN CAMS are causes of a change of timing on all types of engines, and are the most frequent cause of reduced valve lift with its consequent trouble of overheating.
If the outline or contour of a cam is changed with wear it should be replaced, if keyed to the shaft, as it will be a constant source of trouble. If the cams and cam-shaft are in one integral piece, it will be necessary to replace the entire shaft.
(h) WORN CAM ROLLERS AND ROLLER PINS will reduce the lift of the valves, and in the case of a broken or sheared pin will prevent the valve from lifting at all. Always replace loose pins or loose rattling roller.
(i) PUSH ROD DEFECTS. Too much clearance between the push rod and valve stem will reduce the lift of the valves and change the timing. The clearance for small engines should be equal to the thickness of a visiting card, and for large engines is somewhat larger, say 1–16″. The increase of clearance is due principally to wear.
Too small a clearance should be avoided for the reason that the valve stems expand with the heat and will lift the valves too soon, or even permanently until readjusted. Broken valve springs will cause trouble, or lost keys that retain the valve spring washers. Loose adjusting screws on the push rods or stripped threads will delay the valve opening.
(j) TAPPET LEVER DEFECTS are generally caused by wear or poor adjustment. Loose pins or bushings, too much clearance between the tappet and valve stem or broken valve springs, or loose adjusting screws will produce changes in the timing or valve lift.