The Curtiss 0X-5

In a class by itself

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.

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