Some machines carry their own distinctive personality — ones about which the “old-timers” always just nod and smile when their name is mentioned. With engines, it’s often a distinctive sound, the way it cranks over and starts up, or how it whines at full power. For many of those who either remember or simply enjoy antique airplanes, the Curtiss OX-5 V-8 engine has that distinctive, unmistakable sound. Admirers and detractors alike agree that “you always know an OX-5.”
Even for its time, the Curtiss OX-5 was neither state-of-the-art nor even close to “high performance” on the airframes to which it was mounted. It required an incredible amount of pre- and post-flight maintenance, and time between overhauls was very short, often less than 50 hours, as recommended by the Curtiss maintenance manual. And yet, after a hundred years, a good number of them are still being flown by pilots and restorers who wouldn’t think of using anything else. So why all the adoration and loyalty?
For one thing, the OX-5 is an engineering and manufacturing milestone in aviation history. Glenn Curtiss was known for assembling a world-renowned engine design team, including Charles Manley, Henry Keckler, and Charles Kirkham. Beginning with its first two-cylinder motorcycle engines that eventually led to aviation power plants, the Curtiss company was dedicated to the V-style cylinder arrangement that grew quickly from two, to four, then to eight cylinders. (Curtiss also developed in-line engine models.)
By 1908, the basic “OX” design was in place. For the next few years it went through several modifications (from OX, to OX-2, and OX-3), especially in valve train design and fuel delivery methods, until 1915 when the first OX-5 was born. The massive (and heavy) aluminum wet sump crankcase was the source of the force feed and spray lubrication system. The Curtiss manual warns that “the proper lubrication of any mechanism is of so vital importance, and any neglect is so sure to cause expensive repairs that the mechanician should make it his unfailing habit to look after it daily.” On the side of the crankcase was both a simple metal pointer attached to a float inside the sump, as well as a manual sight displaying the level in the 3-gallon sump chamber. Two baffle plates inside the crankcase sloped toward the center from each end providing positive lubrication and “at no practical flying angle can the cylinders become flooded with oil.” A simple gear pump forced oil from the sump to the hollow camshaft and bearings, then to the crankshaft bearings via connecting tubes. Oil spray flung off the crankshaft lubricated piston pin bearings and cylinder walls.
Eight cast-iron cylinders were individually bolted to the rigid crankcase and arranged in two banks of four in a 90-degree “V.” Each cylinder was held in place by four long bolts attached (in most cases) to cross strapping on top of the head. The style created cooling challenges which was met by designing cross-flow cylinder heads with overhead valves and water cooling. The first models used a copper-nickel alloy water jacket which soon gave way to one of brazed-on steel.
Keckler developed a unique valve operating system which used one nickel steel intake and one tungsten steel exhaust valve per cylinder. The exhaust valves required an unusually long rocker arm actuated by a solid steel push rod that moved up and down inside the larger hollow intake push tube. The unique and sometimes fragile valve mechanism used no internal oil and, in fact, had to be hand lubricated before each flight. As the Curtiss manual described, “minute holes are drilled in alignment with the external holes. As oil is forced into these external holes [by hand] the hollow spaces inside the pins act as reservoirs, and will oil evenly all bearings of the rocker arm mechanism for several [usually no more than three] hours after being filled.” This arrangement allowed for greater valve lift and duration for the “under square” (thus high torque) engine with its 4-inch bore and 5-inch stroke. The OX-5 displaced 503 cubic inches and created 90 hp at 1,400 rpm. It was the best Curtiss engine design yet and would serve the aviation industry in large numbers for the next 30 years.