The Macomber engine had seven cylinders of 7.00 in bore and 4.25 stroke, with cylinder axes parallel to the central shaft; but it differed from other engines of its class in that the length of stroke and the compression ratio could be varied with the engine idle or running. This was accomplished through changing the angle of the stroke-plate by means of a rack and gear control. The entire engine revolved with the exception of the small case at either end. Ball and socket joints at each end of the connecting rod joined the pistons and the stroke-plate or wabble plate, that revolved with the cylinders. The central shaft and wabble plate were carried on ball bearings.

The cylinders were individual grey iron castings with integral cooling fins. The valves were placed in an inclined position in the heads of the cylinders, being in a plane through the axis of the central shaft, with the exhaust valve to the outside. Each pair of valves was operated from a single rocker pivoted between the two and with one end sliding in a grooved four point cam located on the central shaft and running in the same direction.

Ignition was provided by a Bosch magneto that was operated directly by the cam gears. The current was taken by about six inches of wire to a stationary electrode on the top of the front bearing case, from which the sparks would jump to the spark plugs in the cylinder heads passing within one-sixteenth of an inch from it. Any standard carburettor or a fuel injection system could be employed as desired. The engine was claimed to have s speed range from 150 to 1,500 rpm. At the normal speed of 1,000 rpm, It was reported that 50 hp was developed. The complete weight was 230 lb and the dimensions were length 28 in and diameter 19 in.