should be returned there immediately after being used. Keeping the gauges and testers clean is about all the maintenance that is required. If they are dropped, broken, or jarred out of calibration, it is generally necessary to return them to the manufacturer for repairs or to replace them.

VALVES, VALVE MECHANISMS, AND CYLINDER HEADS SERVICING

When an engine has been properly maintained and serviced, the first major repair job it will need will normally involve the valves. A general procedure for servicing valves is described in the NAVEDTRA training manual for second class Construction Mechanics. Here, you will get more details on the servicing and troubleshooting of valves, valve mechanisms, and cylinder heads.

VALVE TROUBLES

Some of the common valve troubles that you may encounter in working with engines, and possible causes of these troubles, are indicated below.

Sticking valves may be caused by gum or carbon deposits, worn valve guides, a warped valve stem, insufficient oil, cold engine operation, or overheating.

Valve burning maybe caused by a sticking valve, insufficient valve tappet clearance, a distorted seat, overheated engine, lean fuel-air mixture, preignition, detonation, or valve seat leakge.

Valve breakage may occur by valve overheating, detonation, excessive tappet clearance, seat eccentric to stem, cocked spring or retainer, or scratches on the stem caused by improper cleaning.

Valve face wear maybe caused by excessive tappet clearance, dirt on the face, or distortion.

Valve deposits may be produced by gum in the fuel, a rich fuel mixture, poor combustion, worn valve guides, dirty oil, or the use of a wrong oil.

VALVE ADJUSTMENTS

Proper and uniform, valve adjustments are required for a smooth running engine. Unless the clearance between valve stems and rocker arms or valve lifters is adjusted according to the manufacturer's specifications, the valves will not open or close at the proper time, and engine performance will be affected. Too great a clearance will cause the valves to open late. Excessive clearance may also prevent a valve from opening far enough and long enough to admit a full charge of air or fuel mixture (with either a diesel or gasoline engine), or it will prevent the escape of some exhaust gases from the cylinder. A reduced charge in the cylinder obviously results in engine power loss. Exhaust gases that remain in the cylinder take up space, and when combined with the incoming charge, reduce the effectiveness of the mixture. Valves adjusted with too little clearance will overheat and warp. Warped valves cannot seat properly and will permit the escaping combustion flame to burn both the valve and valve seat.

When reassembling an engine after reconditioning the valves, make sure the adjusting screws are backed off before rotating the engine. A valve that is too tight could strike the piston and damage either the piston or the valve, or both. Adjust the valves according to the manufacturer's specifications, following he recommended procedure. On any engine where valve adjustments have been made, be sure that the adjustment locks are tight and that the valve mechanism covers and gaskets are in place and securely fastened to prevent oil leaks. Overhead Valves Most overhead valves are adjusted "hot"that is, valve clearance recommendations are given for an engine at operating temperatures. Before valve adjustments can be properly effected, the engine must be run and brought up to normal operating temperature.

To adjust a valve, remove the valve cover and measure the clearance between the valve stem and the rocker arm. Loosen the locknut and turn the adjusting screw in the rocker arm, in the manner