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AS engineering standards have risen, the need for production ingenuity has become greater than ever before. The engineer looks to the shop for major assistance in realizing his ideals of improved products. He expects the shop voluntarily to reduce the variations from dimensional specifications and to improve its facility to meet changes in design. Refinement in design is useless unless the shop can accurately hold the dimensions. Powerplant characteristics are largely controlled by the accuracy of centers and roundness and straightness of bores in cylinders and bearings. Crankshaft balance, quiet valve tappets and uniformity of weight and fit of reciprocating parts are all dependent upon accuracy of machine operations. To be able to make design changes in the product without great expense is vitally important. Tools must be designed with facility for change. Fixed-center boring machines are to blame for considerable engine trouble and may make design changes prohibitively expensive.

SIX CYLINDERS are used in the Chevrolet engine, because six cylinders give smoother action and a longer range of satisfactory performance than four. Maximum results per dollar has been the ideal in the design, and high output has been secured at a cost very little higher than for a four-cylinder engine. The piston displacement is large enough to give satisfactory performance without fine tuning. The bore is made as large as possible within the space required for water-cooling around the valves. The stroke is short, resulting in low inertia forces and a stiff crankshaft with the minimum amount of metal. Three main bearings are found sufficient, because of the stiffness of the shaft and the inherent balance of the groups of three cylinders. Positive lubrication is provided, without pressure. The overhead-valve mechanism is so proportioned and the cooling of the parts is so arranged that variations in expansion cancel each other and result in nearly constant valve clearance.

COMPARISONS are made of the respective characteristics of the large-bore short-stroke engine and the small-bore long-stroke engine in connection with the argument of the author that the former engine best fulfills the requirement that an engine must be a good product that is easily produced. He chooses the L-head type of engine for purposes of illustration, since this type is within the scope of the experience of all automotive engineers. When consideration is being given the specifications of a new engine, the first problem to be met is the determination of length. Usually a certain length is set arbitrarily, but this circumscribes the designer at the outset and, for some unaccountable reason, a new project is thus compromised rather than to change the preconceived idea of what the length of wheel-base must be.