72 SCIENCE OF SUCCESSFUL THRESHING. 



a valve, he finds that it has been disturbed since it left the 

 testing room. Do not, then, conclude that your valve is 

 "off," until you have carefully investigated whatever trouble 

 there may be. 



There are men in nearly every locality throughout 

 the country, who are confident that they themselves 

 know more about setting valves than do the manufac- 

 turers. These men affirm that whatever trouble they may 

 have is due to the working of the valve, and, when no im- 

 provement is shown after they have re-set it, they say that 

 the valve-gear was not properly constructed and designed 

 originally. If they had carefully investigated the trouble 

 before disturbing the valve, they would have discovered the 

 real cause, due probably to either insufficient cylinder and 

 valve lubrication, or to a priming tendency of the boiler. 

 The causes of, and the remedies for these difficulties are dis- 

 cussed elsewhere in this book. 



Finding the "Dead Centers." An engine is on its "dead- 

 center" when a line drawn through the center of the piston- 

 rod will pass through the center of the crank-pin. There 

 are two, the "crank" dead-center, when the piston is at the 

 end of the cylinder nearest the crank-shaft, and the "head" 

 dead-center, when the piston is at the opposite end. An 

 engine is said to be running "over" when the top of rim 

 of fly-wheel runs away from the cylinder and running 

 "under" when the top of rim of fly-wheel runs towards the 

 cylinder. "Case" engines are marked for finding the dead- 



