The chemical action of the exhaust gases, especially when any considerable amount of sulphur is contained in the fuel oil, rapidly forms pits that enlarge to some magnitude. This manner of pitting is responsible for the majority of leaking valves. It can be largely offset by more rigid fuel specifications, which exclude all oils having a sulphur content above 1 per cent. and all oils possessing even a trace of acid.
Cleaning Valves.—Once a month each valve and cage should be removed for inspection and cleaning. A regular schedule can be followed whereby the valves of one cylinder are removed each week; in a four-cylinder engine this gives a monthly inspection, while in a three-cylinder engine an inspection every three weeks is secured. When the engine is a single- or double-cylinder unit, the schedule should be arranged so that the monthly examination is obtained. At these inspections the valve and cage should be completely disassembled and thoroughly cleaned with kerosene, washing off with gasolene. If a valve and cage is kept on hand, the old one can be cleaned at leisure. This spare set is very essential where an engine operates almost continuously; The time. required for removal and re placement of cage and valve should not exceed thirty minutes when the engine room force is well organized.
Grinding Valve s.—If the valve seat becomes rough, allowing the com pression to escape, it must be re ground. Where the valve seats in a cage the unit is disassembled. The valve and dashpot or spring cap, as it is more popularly termed, is re placed in the cage with a light spring resting between the valve body and the stem housing, along the lines of Fig. 105. The cage is inverted, placed on some form of support, and the valve pin wrench is then set in position. The spring raises the valve off the cage seat until a slight downward pressure is exerted by the man doing the regrinding. A mixture of emery flour and oil or emery flour and vaseline should be coated over the valve face and the valve rotated back and forth. The valve should, in no case, be completely revolved; the rotation or movement should cover a trifle more than a quarter circle. After rotating a few minutes, the valve should be moved another 90 degrees and the rotation renewed. This grinds every por tion of the valve face to conform to the entire seat of the cage.
As the operator grinds the valve, he should release the down ward pressure on the valve, allowing it to rise from the seat. This serves to distribute the emery paste over the entire face. If this is not observed, the paste forms at the edges only, causing the valve seat to be ground concave.
The operator need not secure a ground seat over the entire valve face. A narrow contact H6-inch wide is ample; in fact, a line contact of 3 2-inch width is as serviceable a gas seal as is a wider space. After the seat is sufficiently ground, the emery paste can be removed. The valve should then be rotated without any paste between the two surfaces; this metal to metal grinding or rubbing will make the area of contact as smooth as a mirror and prolongs the tightness of the valve.
When the valve becomes so pitted or grooved that grinding will not be of any avail, the cage and valve faces must have a light cut taken off their surfaces. The valve can be centered in a lathe and a finishing tool used, making the cut as light as possible. To reface the cage a refacing machine, similar to a rose reamer, is necessary. This reamer must have a stem resting in the valve-stem housing to hold the reamer square with the valve stem. Most manufacturers are in a position to supply this machine, although any machine shop can build one like the sketch in Fig. 106. The cutting head should be of tool steel while the stern can be either of tool steel or machine steel. The latter is preferable since the cutting head can be hardened and the machine-steel stem turned to bring the cutter concentric.
Few plants possess any stand to hold the cage other then a wooden box. The box is a poor accessory since it is almost impossible to seat the irregular-formed cage on it with any feel ing of security. A wooden grinding stand, similar to Fig. 107, can be made during odd hours and certainly repays all labor spent on it. The opening in the top should conform to the shape of the cage while the two stud-bolts should fit into the regular stud-bolt holes in the cage. With such an apparatus an engineer can sit down while grinding, thus lessening as much as possible the labor involved in this operation.