It may be mentioned with regard to the lump on the opening side of the exhaust cam, that this if overdone is found to be detrimental on large engines, and even on small ones. If it is too large, it will cause both exhaust valve and seat to become burnt and pitted, due to the surface being exposed to the exceedingly high temperature of the expanding gases. If it is too large, it is equivalent to opening the exhaust valve too early, and the effect is the same, viz., a waste of power and damage to the valve and its seat.
Fig. 43.
Fig. 44.—Brake Testing.
The method of grinding in the valves to their seats with emery powder and oil is so well known that no further description is needed here. We give, however, in fig. 43 a sketch showing a very expeditious way of dealing with very badly worn or burnt seats. The sketch explains itself. Such a tool is readily made; even the cutter could be turned and filed up to shape and then hardened at home. By lightly tapping in the taper cotter pin little by little, sufficient pressure is put on the cutter to make it an easy matter to completely re-face an old seat or form a new one. A T-wrench or "tommy" can be used to work the cutter spindle. The lower part of the latter must be the same diameter as the existing valve spindle; the bush acts as a guide; and as the bevel of the cutter should be the same as that of the valve, a very little grinding in with emery powder is required to finish the job off.
In fig. 44 we give a diagram showing the method of testing for Brake H.P. of engine, as it is frequently interesting to make such a simple test after any alterations or adjustments have been made.
Two spring balances and a rope or cord (according to the size of the engine), fitted with a few wood blocks as shown in section, fig. 44, to keep the rope on the rim of fly-wheel, is all that is required for this test. The following formula may be used for arriving at the B.H.P.:—