E IN 1938 it became apparent to the Aircooled Motors Corporation that the trend of the power requirements for the smaller airplanes would be constantly toward higher limits. A long range engineering program was undertaken with the idea of eventually having a series of engines in production at an economical cost with a power range from 65 to possibly 400 hp. As yet the sales volume of the light airplane is such that it does not permit large expenditures for mass production of the airplane or engine. For this reason, the engine company is faced with the problem of making their designs flexible enough so that several models can be produced on a basic set of tools. This fundamental has been followed in the design of the power plants mentioned above. The importance of volume production is shown in Stout's statement that the airplane field today needs a 100 hp. engine weighing 100 lbs. for $100 at a production of 100 a day. I t is felt that this is possible to meet, providing there is a volume of 100 a day and when the $100 refers to the labor and material bill of the engine maker and not to the selling price of the engine. After arriving at the definite sizes for the projected series of engines, further studies were made to determine type or shape of these engines. Considering various types from all angles it was concluded that the horizontal opposed type of from four to twelve cylinders would work out to best advantage for our particular type of plant equipment. Briefly, this decision was based on the fact that the Aircooled Motors Corporation had had experience in building the horizontal opposed type on their 50 hp. engine, the power plant was compact, economical to build and very simple from a lubrication standpoint, inasmuch as dry sumps, etc., would be eliminated. Furthermore, the horizontal opposed type could also be readily converted for applications other than aircraft by the addition of cooling fan, air housing, flywheel housing, etc. The first consideration in any engine after the particular type and size is determined, is the crankcase. Various cases were designed—that is, the barrel type of case in which the shaft would enter from one end, the horizontal split type of case where the separation passes through the center of the cylinder pilot holes, and the final solution was to make the case in two halves, splitting it vertically through the crankcase axis. This type permitted the use of a separate oil sump which could be easily attached and varied according to the particular application. Also, in it was incorporated a separate crankcase cover which would permit various types of mountings, accessories, etc. (See Fig. 1.)