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permit the valve head to rest against the seat. While the condition is exaggerated in the sketch it will be apparent that unless a definite space exists between the end of the valve stem and the valve lift plunger that grinding will be of little avail because the valve head will not bear properly against the abrasive ma- terial smeared on the valve seat.
When a bit stock is used, instead of being given a true rotary motion the chuck is merely oscillated through the greater part of the circle and back again. It is necessary to lift the valve from its seat frequently as the grindijig opera- tion continues, this is to provide an even distribution of the abrasive material placed between the valve head and its seat. Only sufficient pressure is given to the bit stock to overcome the uplift of the spring and to insure that the valve will be held against the seat.
The abrasive generally used is a paste of medium or fine emery and lard, oil or kerosene. This is used until the surfaces are comparatively smooth, after which the final polish or finish is given with a paste of flour emery, grindstone dust, crocus or ground glass and oil. An erro- neous impression prevails in some quar- ters that the valve head surface and the seating must have a mirror-like polish. While this is not necessary it is essential that the seat in the cylinder and the bevel surface of the head be smooth and free from pits or scratches at the completion of the operation. All traces of the emery and oil should be thoroughly washed out of the valve chamber with gasoline be- fore the valve mechanism is assembled and in fact it is advisable to remove the old grinding compound at regular inter- vals, wash the seat thoroughly and sup- ply fresh material as the process is in progress. The truth of seatings may be tested by taking some Prussian blue pig- ment and spreading a thin film of it over the valve seat. The valve is dropped in place and is given about one-eighth turn with a little pressure on the tool. If the seating is good both valve head and seat will be covered uniformly with color. If high spots exist, the heavy deposit of color will show these while the low spots will be made evident because of the lack of pigment. The grinding process should
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be continued until the test shows an even bearing of the valve head at all points of the cylinder seating.
Piston Troubles
If an engine has been entirely disman- tled it is very easy to examine the pis- tons for deterioration. While it is im- portant that the pis|ton be a good fit in the cylinder it is mainly upon the piston rings that compression depends. The piston should fit the cylinder with but little looseness, the usual practice being to have the piston diameter at the point where the least heat is present or at the bottom of the piston. It is necessary to allow UiOre than this at the top of the piston owing to its expansion due to the direct heat of the explosion. The clearance is usually graduated and a pis- ton that would be .005-inch smaller than the cylinder bore at the bottom would be about .0065-inch at the middle and .0075- inch at the top. If much more play than this is evidenced the piston will "slap" in the cylinder and the piston will be worn at the ends more than in the center. Pistons sometimes warp out of shape and are not truly cylindrical. This results in the high spots rubbing on the cylin- der while the low spots will be blackened where a certain amount of gas has leaked by.
Mention has been previously made of the necessity of reboring or regrinding a cylinder that has become scored or scratched and which allows the gas to leak by the piston rings. When the cylin- der is ground out, it is necessary to use a larger piston to conform to the en- larged cylinder bore. IMost manufactur- ers are prepared to furnish over-size pis- tons, there. being four standard over-size dimensions adopted by the S. A. E, for rebored cylinders. These are .Oio-inch, .030-inch, and .040-inch larger than the regular dimensions. Care should be taken in reboring the cylinders to adhere as closely as possible to one or the other of these standards.
Rcinoving Pistons Stuck in Combustion Chamber
The removal and replacement of pis- tons and rings seldom offer any trouble
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