In removing the valve cages, the caps of the pedestal bearings f, Fig. 86, are first unbolted and a rope sling passed around the ends of this rocker shaft. Since the shaft is lifted, the entire valve rocker mechanism for this particular cylinder is removed. While this appears to be a more difficult task than the removal of part of a rocker arm as in Fig. 80, actually it consumes no more time and, if the plant possesses a traveling crane hoist, is no more difficult.
Valve Cam Clearances.—The roller clearances to be main tained are as follows: • Admission valve cam clearance 0.039 inch Exhaust valve cam clearance 0.036 inch Fuel valve cam clearance 0.002 inch Starting valve cam clearance 0.036 inch Figure 88 gives the valve timing which is correct for the 500 h.p. engine at 164 r.p.m., and this equally applies to all sizes of the slow-speed engine. The manufacturers set the admission and exhaust valves on the erecting floor and do not recommend any change. The helical gears are marked to enable the erector to assemble the valve mechanism. Nevertheless, after a few years of service the gear teeth and the cams wear to such an extent that the valves do not function as originally set. It then becomes necessary to retime the valves along the settings indicated above.
The rocker shaft is provided with small grease cups. Usually the lubrication so obtained is insufficient, and the cups should be replaced by sight-feed oil cups.
Snow Diesel Engine Valves.— Figure 89 outlines the admission valve while Fig. 90 is the exhaust valve assembly of the Snow engine. The valve stems are identical, while the valve bodies differ in that the exhaust valve body is extended along the stem, forming a gas deflector to avoid the burning of the stem. The valves have one feature that makes them rather peculiar—the valve seats are flat surfaces. The advantage of the flat valve lies in its freedom from any wedging action, with the consequent grooving which, at times, occurs with the bevel seat, and in the increased valve opening over the bevel seated valve for any given lift. There is, however, an objection to the flit valve which an occasional engineer voices. It is much more difficult to keep gas-tight, largely due to the accumulation of small carbon particles on the face. The valves are easier to regrind, and con
sequently any deposits can be quickly removed.
The valve stem is equipped with labyrinth grooves, and it becomes necessary to clean these recesses of tar each time the valve is inspected. Since the side pressure from the rocker arm is against the stem, the stem must be copiously lubricated; this is obtained by a line from the mechanical oil pump on the engine frame. The cage is sealed by a ground joint at the inner end as well as by a gasket at the outside flange.
Camshaft.—The camshaft, as will be noted in Fig. 91, is driven by bevel gears from the engine layshaft. The rockers are fulcrumed on a short shaft which is supported by the cylinder head. To remove a valve cage, the entire rocker assembly must be unbolted. This fulcrum shaft is provided with an eccentric bushing upon which are fitted the starting and fuel valve rockers.
Cam Clearances.—The proper roller clearances are as follows: Admission valve cam clearance 0.01 inch Exhaust valve cam clearance 0.01 inch Fuel valve cam clearance 0.004 inch • Air starting valve cam clearance 0.036 inch The proper valve timing appears in Fig. 92. Any change of the entire cam timing can be obtained by shifting the gears one or two teeth, while the alteration of an individual valve cam can be secured by the use of a properly offsetted key.
McEwen Diesel Valves.—One excellent feature of this hori zontal engine is the provision of cages for both the exhaust and the admission valves. These cages and valves are mounted in the cylinder head in vertical positions, as appears in Fig. 70. Both valves are of cast iron with steel stems; the exhaust valve body is extended to form a hood about the lower part of the stem.
The two valves and the admission valve cage are shown in Fig. 93; both valves have 45-degree seats. To remove either cage, it is only necessary to unship the reach-rod pin and lift out the cage with the rocker attached.
The camshaft is driven by a helical gear from the engine shaft. The valve rocker arrangement is outlined in Fig. 70. Each valve has its individual cam, the exhaust cam roller being provided with a shifting pin which relieves the compression on starting the engine.