68 SCIENCE OF SUCCESSFUL THRESHING 



ject of making the valve in this way is to equalize com- 

 pression. Such a valve should be put in with the wide 

 end toward the rear or crank-end of cylinder. 



The Piston. The piston is always a little smaller than 

 the inside diameter of the cylinder. It is made steam 

 tight, however, by rings which are fitted into grooves on 

 its circumference. These rings are originally made 

 slightly larger than the bore of the cylinder, and are after- 

 ward cut apart, so that they may be compressed sufficiently 

 to enter the cylinder. This gives them some tension so 

 that they fit the inside of the cylinder closely, thus pre- 

 venting leakage of the steam. The cylinder is bored 

 slightly larger at the ends "counter-bored" as it is called. 

 This is done to guard against the wearing of a shoulder, 

 at the points, near each end of the cylinder, at which the 

 outer edge of the piston ring stops. The forming of such 

 a shoulder (which would cause the engine to pound), is 

 prevented by allowing part of the ring to pass into the 

 counter-bore. The entire width of the ring must not be 

 permitted to enter the counter-bore, however, or the ring 

 would expand and catch against the shoulder. 



To Divide the Clearance. The clearance of an engine 

 is the cubical contents of the port, from the face of the 

 valve to the cylinder, including the space between the pis- 

 ton and the cylinder head when the engine is on dead-cen- 

 ter. To divide the clearance, loosen the clamp bolt and 

 the jam nut on the piston rod and unscrew the rod from the 



