ROUGH-AND-TUMBLE ENGINEER 79 



inakc your engine bump milar to that caused 



by too much or too little "lead." The only remedy for 



a wheel of this kind is to have a new key made, and be 



he new key fits the key scat in both the whcd 



and sha 



LEAD 



\\ ' id? Lead is space or opening between the 



and the edge of the port on the steam end of cylin- 

 der when the engine is on dead center. (Dead center 

 e two points of disc or crank wheel at which the 

 crank pin is in direct line with piston and at which no 

 amount of steam will start the engine.) Different makes 

 of engines differ to such an extent that it is impossible 

 to give any rule or any definite amount of lead for an 

 engine. For instance, an engine with a port six inches 

 long and one-half inch wide would require much less lead 

 th a port four inches long and one inch 

 . Suppose I should say one-sixteenth of an inch 

 was the proper lead. In one engine you would have 

 an opening one-sixteenth of an inch wide and six 

 inches long and in the other you would have one 

 one-sixteenth of an inch wide and four inches long: 

 so you can readily sec that it is impossible to give the 

 amount of lead for an engine without knowing the piston 

 area, length of port, speed, etc. Lead allows live steam 

 to enter the cvlinder just ahead of the piston at the point 

 n the stroke, and forms a "cushion," and en- 



