This article appeared in the March 1998 issue of Boat & Motor Dealer.
After spending the better part of 30 years involved with the design of outboard and stern drive working parts and trying to improve the service life of parts that had failed with too great a frequency in the field or in endurance testing, I cannot help but wonder at the proliferation of critical service parts that are now available from sources other than the engine manufacturer. 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. Time in the carburizing process is controlled to allow carbon penetration to a "case" depth of 0.025 to 0.040 of an inch, and the carbon content is raised to produce a very hard, fine grain surface (60-65 Rockwell C scale, when the parts are cooled). It is also important to control the cooling process to produce the right core hardness for a tough but more ductile core (30-45 Rockwell C scale). A high carbon content all the way through would result in a brittle part. The gears are then finished by precision-grinding the bore and thrust surfaces. it was known that using gear tooth surfaces that were machined resulted in some minute distortion after heat treating and an imperfect tooth contact or "pattern."
To get the most load carrying ability out of a given size gear, it was necessary to finish grind the tooth surfaces after heat treatment. Another process called "shot peening" could be applied, in which the tooth surfaces and root area were bombarded by extremely hard carbide particles, which closed up the surface grain structure and placed it in a compressed state. This greatly increased resistance to fatigue failure, which can occur when a gear tooth is exposed to millions of cycles of high stress.
When a gearcase of a powerful outboard or stem drive is exposed to high power and torque inputs from the driving engine, it doesn't remain straight as it was when machined, but bends and deforms under the load-not permanently, but during the time when under load. Looking at the gearcase from ahead, the lower "bullet" will move to the side a measurable amount relative to the driveshaft centertine, perhaps as much as 0.020-0.040 of an inch. This affects the gear tooth contact pattern between the pinion and gear and moves it off the center of the tooth face. The cutting and grinding of the tooth surfaces need to be adjusted to compensate for this gearcase deflection and to keep the contact pattern centered.
These details are contained on the engine manufacturer's drawings and part specifications, which are proprietary information and are not available to outside manufacturers of "duplicate" service replacement parts. The same appearing gear can be made from different alloy steels, with different case depths, surface and core hardness, and tooth cutting and finishing precision.
Corners can be cut to keep costs down and still produce parts with a "reasonable" life expectancy, which may be sufficient if the engine is not used too much at maximum power. Maybe they can do all the things the engine manufacturer has done to produce a product with a life expectancy the customer anticipates, but I wouldn't take this for granted. It costs money to have the equipment and testing facilities that are necessary to make the best possible gears for a lower unit and power.
And that's just the gears. Similar stories can be written for propeller shafts, driveshafts, crankshafts, connecting rods, pistons and other critical and highly stressed parts, even water pump impellers. Can the manufacturer's expertise and experience be duplicated by outside parts sources? Maybe, but I doubt it.
Ralph Lambrecbt is an engineer with more than 40 years experience in the marine industry and marine safety standards development.
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