Cam Clearances.—The valve cam clearances follow: Admission valve cam clearance 0.005 inch Exhaust valve cam clearance 0.03 to M2 inch Injection valve cam clearance 0.001 inch Starting valve cam clearance 0.03 to ;62 inch Valve Timing.—The timing diagram applicable to the engine appears in Fig. 94. These values can be followed by the opera tor with a feeling that they are correct.
National Transit Diesel Valves.—This engine has the admis sion valve provided with a cage while the exhaust valve has a half-cage, which carries the valve spring and stem housing. The latter valve does not seat on the cage but instead a removable seat is set into the cylinder casting and held in place by machine screws, Fig. 95.
The valves differ in that the exhaust valve body is extended to form a guard around the stem. In removing the valves, the admission valve cage is first lifted off, and then the exhaust valve spring and cap are unshipped. The exhaust valve can then be raised through the admission valve cavity. If it is desired to remove the exhaust cage, a rope can be passed through the valve stem housing and tied; slipping the other end through the admis sion cage cavity and hooking it to a hoist, the exhaust cage may be lowered to the floor or lifted into place as the case may be. The false exhaust seat is difficult to remove when worn as the screw threads tend to burn and corrode fast. Vigorous ham mering with a long bar will frequently loosen the screws; kero sene should always be poured on the seat since this will assist in cutting away the iron rust from around the screw threads.
Valve Cam the valves are actuated by wiper type levers. These levers are controlled by a single eccen tric as shown in Fig. 95. Since the contact between the flat valve lever and the rocking levers is a rolling one, the clearances are not of such serious moment. The correct timing of the valves can be secured by either shifting the eccentric, or altering the length of the valve stems through the agency of the valve lock nut caps, or by adjusting the length of the valve reach-rods. The latter is the proper method for securing any minor timing adj ust ments. The valve timing of this make of engine appears in Fig. 96. It is apparent from Fig. 13 that the air starting and fuel
injection cams are distinct from the valve eccentric.
National Transit Diesel, 1918 Model Valves.—The later en gines produced by the National Transit Co. have a valve and camshaft design radically different from the one discussed above. The valves are placed horizontally in the cylinder head, and both are provided with cages. This entire assembly appears in Fig. 96A. The end of the valve stem has an adjustable head which is in contact with the valve rocker, and a collar against, which the valve spring rests. No spring cap or other bearing is provided for the support of the outer end of the stem. However, the cage sleeve is long and gives ample support.
. Camshaft Layout.—The rocker arms fulcrum on a short shaft which is carried on bearings bolted to the head; this shaft is loose in the bearings and is held by the starting lever shown. The rocker rollers rest on the cams which are keyed to the short cam shaft; this shaft is driven by the layshaft through bevel gears.
The advantage of this design lies in the better cylinder head that is permissible with horizontal valves.
Valve Timing.—The timing of the valves is the same as applies to the former design and appears in Fig. 96.
Allis-Chalmers Diesel Valves and Rockers.—The valve ar rangement of this engine, Fig. 97, is quite similar to the one just discussed in Fig. 95. The exhaust valve, however, has no re movable seat since it rests directly on the cylinder-head casting. Use is made of a single eccentric for both valve actuating rods. In the two-cylinder engines a system of links allows the one camshaft to control the exhaust and admission valves of both cylinders. The valve rockers have a wiping action and conse quently open and close the valve, while the rocker is moving at a maximum speed, without shock or noise. The noiseless valve action is especially noticeable to one accustomed to the slam ming noise of the usual cam-controlled Diesel valves. Since this form of valve actuating levers is used, no clearance is required between the lever and valve cap when the eccentric is in its extreme low position, beyond that necessary for the oil film, which need not be more than .001 inch.