The Reid Model A gas engine was one of the very few small engines to make a commercial success of the Clerk cycle. This differs from the common two-stroke cycle in that the scavenge pump leads the power cylinder by only 90 degrees, and therefore must consist of a second cylinder with its own crank throw. In the Reid, the second throw is a pin set in one of the flywheel spokes, and the pump rod works outboard of the wheel. Intake and transfer are through automatic poppet valves under the head; exhaust is through ports. The later timing of the pump reduces the loss of fresh charge through the exhaust ports, while allowing higher compression in the pump with some supercharging of the power cylinder. These engines run much more smoothly under light load than a two cycle.
The horizontal, belt driven governor head controls individual spool valves for gas and air. Both hot tube and Wico OC high tension snap mag are provided for ignition. The mains are chain oiled from reservoirs in the frame, and the rod bearing has dual oil feeds through a wick and a centrifugal collector ring. A safety valve in the head is supposed to blow in case of a backfire into the scavenge pump. Although nearly all engines found run on natural gas, a gasoline attachment was available, and the catalogue shows a vaporizing oil model.
The Model A was introduced in 1898 by the Joseph Reid Gas Engine Co, Oil City, Pa. It was built in sizes from 6 to 40 HP, primarily for use in the oil fields. Although a few more modern two and four cycle engines were built in later years, there was no change in the design of the old Model A till the company went out of business some time in the 1930's.