It is one thing to make replacement gaskets and similar components, and another to produce equivalent parts such as highly stressed gears, shafts and other internal engine components that operate under load at high speeds and temperatures. The design of these parts has evolved over a long period of time through trial and error, material and process changes, and thousands of hours of testing by the engine manufacturers. Lower unit gears are one example. These can be produced by a variety of processes resulting in what appear to be equivalent parts. Unless the best procedures are followed, the results will not be equivalent.
Most gears start as a piece of alloy steel bar stock.
Forged gear blanks can also be used. The steel alloy is critical to the final product, a typical one being SAE 8620, which contains about 0.2 percent carbon, 0.8 percent manganese, 0.25 percent silicon, 0.5 percent nickel, 0.5 percent chromium, and 0.2 percent molybdenum, plus trace amounts of sulfur and phosphorus. The gear blank is without teeth, which are cut on a Gleason or equivalent gear cutting machine. The gear blanks are then heat-treated in a carburizing atmosphere, which can be a hot gas or liquid salt bath containing concentrated carbon. A high carbon content all the way through would result in a brittle part.