William Wynne

"The Corvair Authority"
5000-18 HWY 17 #247
Orange Park, FL 32003

Converting Corvair Engines for Experimental Aircraft Manual

The Corvair Flight Engine Conversion Manual by William Wynne, Part No. M-1, is the recognized bible of Corvair conversions. The Manual guides you through selection and purchase of core motors, which are readily available, to engine conversion, installation and engine systems. The 2005 edition contains three times more information than prior editions, as well as many photos, drawings and graphs.

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From The 2005 Corvair Conversion Manual

Cam Gears

Every rebuilt motor needs a new cam gear. The cam gear is a one shot deal on a Corvair and cannot be removed and replaced. If it has to come off, throw it away. It is heated and pressed fit onto the cam. Clark's offers this service, and since you're getting the cam from them, you might as well have them do it for you. They have steel washers and keys; have them assemble your cam with these. They have recently had a brand new run of cam gears made, and they appear to be of excellent quality. In years past, some of the aftermarket gears out there were of questionable quality.

There is an optional gear called a failsafe gear. It is a very high quality billet gear with a steel sleeve heat shrunk over the end. It is nice, but not necessary in your average Corvair flight engine. If you are building a reverse rotation motor, as perhaps 1% of the builders are, special attention needs to be paid to the cam gear, and it needs some type of mechanical fastener holding it on. Bob Sutcliffe is a good source for this modification. On the standard rotation motor, this is not required because the cut of the gears is constantly trying to push the gear on.

Delta Cams

A number of people have expressed an interest in the Delta cam from the Pacific Northwest. This company regrinds a stock Corvair cam. They can do the regrinding without the cam gear being removed. While at first the $50 price combined with keeping the old cam gear sounds like a significant savings (an OT-10 with a new cam gear costs about $130), I think this is a false economy. I do not know of anyone who has run or flown one of these cams, far less tested it against others.

Isky Cams

Isky makes a cam called a "260." Many people say that this cam is a replacement for an OT-10; it is not. The 260 is a single pattern cam, just like the stock cam. This is typical of 1960s designs. If you look at any modern cam catalog, you will see that dual pattern cams dominate all types of modern thinking on cams. A dual pattern cam has a different lobe on the intake and exhaust. This might seem like the obvious way cams should have always been made, but it was not popular until the 1980s.

Offset Keys

There are offset keys available to fine tune exactly the placement of the cam. I have never found the OT-10 to be critical on this issue. All the motors I build in my shop use straight keys. This puts an OT-10 in about 2 degrees advanced, which is how it was designed to operate.

Stock Cams

Bernie Pietenpol tried several different Corvair cams during his flight testing. Of all the cams, he felt the 95hp cam was the best. Keep in mind that his conversions only put out about 70hp. Today we have much better cams available, and no one should use a stock cam. Stock cams do not have enough lift to make good power.

Roller Lifters, Steel Cams

Two groups in California claim to have a reliable roller cam for a Corvair motor. Neither of these people are familiar with aviation engines. I find that the word reliable means something different when you're talking about flying it. The people who offer these cams make exaggerated claims as to their benefits. I have heard them claim that a 20% increase in horsepower is possible. This is pure bull at the rpm we are speaking of. Roller lifters must be run on a steel cam (stock cams are cast iron) and this makes them expensive. The Corvair case is crowded and does not have the room required for an effective link between pairs of lifters. The methods they use to get around this are not proven to be reliable to our standards.

New vs. Reground

The OT-10 is available as a regrind or a new cam. The new cam costs about $50 more. I have flown and tested both of them. On some non-Corvair motors, reground cams make a mess out of the valve geometry and require longer pushrods for the motor to work effectively. This is not so with the Corvair motor, which has far less critical valve train geometry. If you're concerned about this, or having slightly better valve geometry, buy a new OT-10. Mark Langford is one of the few engine builders who opted for the new cam. All the production engines in my shop use reground cams.

Pat Green First To Fly An OT-10

Pat Green, a Pietenpol builder and flyer from Jacksonville, Fla., was the first guy to fly an OT-10. Pat has more than 500 hours on his Corvair-powered Pietenpol. He has a fairly standard, but very clean, Bernie conversion. When he stopped by my shop in 1997, he was looking for more power. I had an OT-10 on the shelf waiting to go in my plane, but was testing a different cam in my plane at the time. Pat took the OT-10 home and called me shortly thereafter to say it worked great. I ordered another one and tested it myself and found out how right he was.

Adjusting Lifters

This is done after the long block is completely assembled, but I include it here because the lifters are part of the valve train. The Corvair requires the hydraulic lifters to be set just once in the life of the motor. If you do it right, you can do this during the rebuild phase, and never have to worry about doing this 6" behind a running prop.

While building the motor, do not put the lifters in oil. When I assemble a motor, I put oil on the lifter body, and Crane cam lube on the bottom. I smear lube all the way around the lobes before the two case halves go together for the last time; it's much easier. To set the lifter, hold it down with your finger and spin the motor over. This will remove excess cam lube that would mess up the setting. After putting the pushrod tubes in and bolting the heads down and putting the pushrods in, you are ready to start.

The lifter has a plunger in the middle held up by a small spring. If it is full of oil you can't compress it, but if it is straight out of the box, you will be able to plunge the lifter about .080". Take a cylinder and make one valve at the middle of its lift. The other one will not be open at all, and is at a good spot to be set. Turn the rocker nut down slowly as you roll the pushrod between your thumb and index finger tip. When the nut gets tight enough, it will begin to plunge the lifter. You will be able to feel it at your fingers because it will not roll very easily.

From this point, turn the nut exactly 3/4 of a turn. Do not pay any attention to what anyone else says about the number of preload turns. I have heard stories of everything from 1/4 to 1 turns, but none of these storytellers will come to your hangar and reset the rockers 6" behind a running prop when their suggestion doesn't work. I have absolutely no evidence that the motor makes any more power at any setting. However, if the motor is set up tighter or looser, it could damage it.

The maximum amount of miles I have gotten out of a land Corvair using this method without further adjustment, when done by my own hand, is about 70,000 in my 1967 coupe between '92 and when I took the motor out of the car in '97 because the body was shot.

The maximum amount of hours I have flown when this adjustment was done by myself to a flight motor, about 350. The motor had the rocker arm balls replaced at this point, but the adjustment was still fine. The maximum calendar time between setting and fire-up on a motor with lifters set this way by me, about 11 months in the case of Jake Jaks motor, but I think it would have been fine for much longer.

I include the "done by my own hand" part because many of the stories you hear on this subject are relayed by someone who has never done it or tested their setting over time. In my experience, relayed info on this subject is erroneous. I consider it a safety issue. I am not bothered by working within inches of a running prop, but many people are rightfully made nervous by it, and if you feel this way, it's a distraction and chances are much greater of getting hurt. Set them right to begin with and remove this risk.

Main Bearings

All Corvair engines use the same main bearings. The J.C. Whitney catalog usually has the cheapest price on these. They come unmarked but are Federal Mogul bearings. Local auto parts stores also carry the Corvair bearings. Always use new bearings in a newly rebuilt engine. Shop around if you get a high price; prices vary greatly on bearings. Previously, I recommended using only stock cranks, but I have since tested .010-.010 cranks and they have given satisfactory results. You must use full flange bearings. There is a thing called a half bearing. It is missing half its thrust face. Do not use one of these sets. Full flange thrust bearings are a complete success story with four decades of flying behind them.

The Corvair has three main bearings which look identical, but one bearing set has a different number stamped on the back. This set goes on the end of the motor. If you miss this, the motor will wear out before its time.


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