GAS, OIL AND ALCOHOL ENGINES 



409 



Piston. The piston (Fig. 292) is one of the important 

 parts of the engine. It should be of good length to carry 

 itself without binding. The piston pin should be near 

 the middle and as long and as large as possible. In 



FIG. 291 BASE 



small engines the piston should be about 1/200 of an inch 

 smaller than the cylinder, and in larger sizes it should 

 be about 1/32 of an inch smaller. The space on the head 

 end of the piston beyond the last ring should be about 

 1/16 of an inch less in diameter than the rest of the 

 piston. 



Piston rings. The number of rings (Fig. 292) varies 

 from three in cylinders of 5-inch diameter and less up 

 to eight in 2O-inch cylinders. If the engine is of the 



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FIG. 2Q2 PISTON AND RINGS 



vertical type, there should be a ring at the lower end of 

 the piston. This ring will prevent "oil pumping." The 

 rings should break joints, and if one edge fits closer to 

 the cylinder than the other, the close-fitting edge should 



