SCIENCE OF THRESHING. 149 



from the head end steam port leading to the valve 

 seat, it clearly indicates that the valve is leaking; if 

 the steam escapes around the piston head, it is plain 

 that the cylinder rings are leaking. 



It is a good plan to test the piston and valve in 

 different positions, as in some positions they are found 

 to leak while in others they are perfectly tight. This 

 is owing to the fact that the cylinder does not wear 

 evenly. 



In testing, great care should always be taken to 

 have the flywheel firmly secured, as play of a few 

 inches might break the chain and do injury to the 

 operator. 



If the valve is found to be leaking, it may be 

 repaired by planing the valve and valve seat, and 

 afterwards scraping to a true surface by means of a 

 scraper and surface plate. 



If the cylinder rings are found to be leaking, they 

 should be expanded or replaced by new ones ; cylinder 

 rings in pistons which are constructed with an inside 

 ring and stud bolts for the purpose of forcing the 

 rings outwardly to the wall of the cylinder, are the 

 only ones that can be expanded successfully. In other 

 forms of pistons and rings where a groove is turned 

 in the piston, and the rings are sprung into place and 

 held by their own elasticity, the usual practice is to 

 replace the rings by new ones. 



In compound engines the same general plan of 

 testing the engine may be followed, but it is some- 

 times more difficult to locate the exact point of the 

 leakage, on account of the multiplicity of the ports 

 and working parts. 



