132 



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 



Popular Science Monthly 



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 



