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.
By 1915, and with war contracts as the incentive, Curtiss began building OX-5s by the hundreds, becoming the first massproduced aircraft engine designed by a U.S. company. At first, they used Schebler carburetors and Bosch magnetos (an option that cost a few dollars more than the standard battery-style ignition). When those became scarce due to wartime shortages, they switched to Zenith carburetors and Berling (later Dixie and Scintilla) magnetos. The OX-5’s 8 gph (at 75 percent power) fuel consumption rate was a plus, though the engine’s heavy weight (like other aircraft engines of the era) of almost 400 pounds resulted in a lack of performance, though the OX-5 could generate 105 hp for brief periods if the rpm was bumped up to 1,800.
But old-timers point out that the engine’s exceptional torque characteristics were more than adequate for turning the long, heavy wooden props of those early birds.
Cooling leaks were a major concern. The engine vibrated so much that each cylinder gasket or water jacket connection was a potential source of loss of critically needed coolant, usually “Prestone,” but just as often plain water. The large capacity cooling system was not forgiving if much of it was lost. The long manifolds and large oil volume made the OX-5 difficult to start in cold weather. Many pilots living in cold weather regions drained the fluids from the engine after each flight and kept the oil and coolant warm inside, refilling them before the next flight. This made preflight preparations long, in addition to all the lubrication with the oil can.
Even filling the radiator could be tricky. As Curtiss described: “Be sure that the water from the radiator fills the cylinder jackets. Pockets of air may remain in the cylinder jackets even though the radiator may appear full. Turn the engine over a few times by hand after filling the radiator, and then add more water if the radiator will take it. The air pockets, if allowed to remain, may cause overheating and develop serious trouble when the engine is running.”
The OX-5 proved to be a perfect fit for the Standard J1 and JN-4 “Jenny” trainers that the Army needed and in which most American-trained pilots got their first taste of “real flight.” Veteran pilots and barnstormers often said, “If you could handle a Jenny [or Standard, or Canuck] you could handle anything else.” Plenty of civilian airframes used the OX-5 such as the Robin, Travel Air, American Eagle, several Waco models, Pitcairn Fleetwing, and many others. Amelia Earhart and Charles Lindbergh logged many of their early flying hours behind an OX-5 engine. As time went by, aftermarket improvements such as dual ignition, roller bearings, improved lubricants, and better carburetors were available to help in performance and reliability.
“Model T” of aircraft engines
After World War I, the U.S. government had a huge surplus of both training aircraft and spare OX-5 engines. Labeled “obsolete” by the military, they were offered to the public at very affordable prices. A “new-in-the-crate” OX-5 went for as low as $20. They were snatched up by barnstormers, air transport companies, flying schools, and individual enthusiasts, creating a whole generation of “golden age” pilots and mechanics learning their first lessons about aircraft on old Jennies, Standards, and OX-5 engines. Because of its availability, low cost and versatility, some people called it the “Model T of aircraft engines.”
The Curtiss team was never satisfied with any design, so it kept perfecting and modifying the OX-5. The 100-hp OXX models, which were basically OX-5s with a larger 4.25-inch bore, were developed as improvements on the original design. Licensing for construction was granted to companies like Liberty Iron Works, St. Louis Aircraft, Willys-Morrow Corp., Canadian Aero, Fowler, Howell & Lesser, Springfield, and U.S. Aircraft. Advertisements for “high performance aircraft assembled from war surplus stock” were published in many newspapers and virtually every aviation-related magazine.
One company, the Nicholas Beazley Airplane Company of Marshall, MO, had a typical mid-1920s ad boasting a “Standard J-1 Commercial” version with a maximum speed of 85 mph, cruising speed of 75 mph, landing speed of 30 mph, and 12,000-foot ceiling. The price (including complete assembly) with a “used OX-5” was $900. A government overhauled OX-5 was $1,000 and a brand new OX-5 was $1,200. (If the buyer wanted a new, used, or rebuilt OXX6 engine, the price increased by about $200 each.)
Aircraft maintenance shops all over the country found there was good money in rebuilding OX-5s, starting a new concept that involved completely overhauling OX-5s and OX-6s. The overhauls would then be put in storage, ready to be installed in less than a day for a customer who flew the airplane in, then waited while a rebuilt engine was pulled from inventory, the old engine removed, then replaced by the rebuilt unit. The original engine (taken in trade) would be overhauled and added to the inventory, waiting for another customer. The only downside was that many aircraft owners may have flown in with a Curtiss-built engine and flown out with one originally manufactured by one of the licensees, which were sometimes slightly inferior in quality. Every shop created its own unique repair techniques and improvements to the engine parts to increase reliability.
Some shops, like one managed by “Murphy” Schedenhelm in East St. Louis, MO, found that with detailed attention paid to each part during overhaul, one of their restored engines could expect 700-1,000 hours between major overhauls and from 200-375 hours between top end (cylinder head and valve system) overhauls.
His team recommended oil changes every 15 hours and the oil screens removed and cleaned every 30 hours. During top end overhauls, the crankcase was pressure tested for leakage by attaching a copper tube from a 2-gallon pressure tank to the engine oil pump connector and circulating oil at 60-70 psi through the lower bearings, watching for excessive leaks. If they determined that a complete overhaul was necessary, crankshafts were ground to .010 to .020 inch undersize and main bearings (and cam bearings) line bored to .0015 inch clearances.
Even though clearances were inspected with Prussian Blue dye, the machinists also checked a lightly oiled re-assembled crankshaft and bearings by giving it a good spin by hand and making sure it turned at least three complete revolutions on its own.
Cylinders were seldom reground. Instead they were replaced along with new rings. Pistons contained no oil scraper rings — compression rings only. Connecting rods and piston pin bosses were refurbished using a Storm boring and alignment lathe (for shops that could afford one). Valve guides could be oversized by 1/64-inch or replaced, and both valve seats and faces could be either refaced or replaced as deemed necessary. Valve springs were always replaced and valve clearances set at .010 inch.
The ever-problematic cooling system was also given careful inspection. Water jackets were pressure tested for leaks and all hoses, connectors, and clamps were replaced, regardless of visual condition. In addition, water jacket outlet gasket surfaces were sealed with shellac. Water pumps were carefully inspected and given new drive shaft bushings and special washers to maintain .002 inch of end play.
During carburetor rebuilds, the float was always replaced and special 1-inch-deep wells were installed below the jets to help trap any water that might find its way into the fuel system.
Magnetos were also disassembled with the armature and condenser tested separately. Bearings were replaced and repacked with a mixture of Vaseline and engine oil with a recommendation of repacking them every 15 hours. A Weidenhoff test stand was used after the magneto final assembly and the distributor gear set at ½-inch retard from normal position.
Once the engine was completely reassembled, it was once again pressure tested for leaks. It was followed by a very strict break-in period. The first three hours the engine was run at 300-500 rpm until the temperature dropped to 140 F. Then another three hours at 800 rpm, watching to make sure the temperature did not exceed 150 F. Then another three hours at 1,000 rpm or until the engine maintained a temperature of 160 F. Then one hour at 1,100 rpm, one hour at 1,200 rpm, and a half-hour at 1,280 rpm. After that the engine was idled for five minutes, run at normal cruise for 10 minutes, and at 1,400 rpm for five minutes. If the engine maintained a temperature between 160-180 F, it was approved for service.
Every maintenance facility had its own variations on this procedure, but varied only slightly. One of the additional advantages of the design was the simplicity of many of the components which, if not available by catalog, could be manufactured and installed with basic skills in most farm machine sheds or blacksmith shops.
While the Curtiss OX-5 and its variants remained in wide service until World War II, its popularity was beginning to slip as new engine designs and heavier airframes were developed by 1928. Curtiss and Wright merged in 1929, marking the end of OX production by the parent company. They began concentrating on more powerful and efficient engines as military contracts once again started driving production.
But there remained a passionate, committed group of OX enthusiasts that continues to this day. In 1955, Cliff Ball formed a group known as the OX-5 Aviation Pioneers in Pennsylvania. Its motto was “Aviation history by those who lived it.” Other groups, clubs, and museums continue to provide venues for OX-5 enthusiasts, historians, and restorers. Today, 100 years after its original design, the Curtiss OX-5 continues to draw crowds at fly-ins where the old-timers don’t even have to look up as one takes off, usually saying, “Yep, you can always tell an OX-5.” And then the stories begin.
Scott M. Fisher is an A&P and received his training at Dakota Aero Tech in Fargo, ND.