Al Leyh applying Clover Grinding Compound to the mounting surface of the cylinder from the engineer's side of the locomotive. The cylinders, like parts of many locomotives, are assembled with a lapped metal-to-metal fit, sans gaskets. The machined surfaces are manually ground down, using successively finer grades of grinding compound. The lapping process goes like this: apply grinding compound(grit mixed with a grease or oil) to one of the mating parts, wiggle or rotate them many times so that the grinding medium erodes away the machining marks until the cutting action stops (due to the grit being crushed or rounded), clean the parts for inspection, and repeat......many times. eventually, by using ever finer grinding compound, the lapped surfaces achieve a VERY smooth surface finish, that when pulled together in tight contact with bolts, forms a joint so precise that steam cannot pass. Bill and Al began with 120 grit, and finished with 500 grit. This process takes hours!

Photo by Bill Liebman