With the successful development of high power, low fuel consumption, air-cooled cylinders it became desirable to investigate whether such cylinders could be cooled with the in-line arrangement. Briefly, advantages of air-cooled compared with water-cooled engines are simplicity, decreased vulnerability from machine-gun fire, freedom from cooling-system leaks, independence of cooling system from extremes of hot and cold weather, and the reduced weight of power plant with fuel and oil economy equal to that of water-cooled engines. This project consisted of the design, construction, and test of air-cooled cylinders assembled on a standard Liberty "12" engine, using overhead cam shafts, and a rotary induction system. Performance and endurance tests were performed. This test demonstrates the feasibility of cooling high-power air-cooled cylinders in a narrow Vee arrangement. The increased fuel consumption during the test was obtained by carburetor mixture control adjustment and was not necessitated by loss of compression which did not occur. Several detail improvements are required and are being made as a result of this test, which will be taken up in the account of the second 50-hour test of this engine.