Above is one of the more elaborate Corvair building tools in our shop. The purpose of this tool is to
test the oil system contained in the rear accessory cover. Normally, the oil pump, relief valve, and
cooler bypass are rebuilt, and the first opportunity to evaluate them is just before starting the engine.
Here, we can clean and assemble an accessory cover with a rebuilt oil pump, correctly shimmed and polished
relief valve, and a stock cooler bypass. This assembly can then be completely tested by temporary installation
on this tool, with the oil pump driven by an electric drill.
Previously, I've tested these systems with a remote gauge pack and a drill prior to the start of every
engine we build. The advantage of the new tool is simple: It allows setting up these systems without the rest
of the engine present. In this way, a builder unsure of his system can mail us his old grimy cover for rebuild,
and receive a completely tested unit in return. The finished unit can then be installed with
absolute confidence. We offer this as a service.
The basic part of the tool is the back end of a Corvair engine. This specific engine is an RL code case
with some history: It is the actual engine from our Pietenpol. The Number 2 bearing bore was about 1/1000"
out of round. So, I decided to ground the case and put it to work as a tool. We have lots of cases in the
shop, so we can be very particular about what we convert into a flight engine. I welded up most of the holes in the case.
We made an oil pan for it with its own pickup. It has a 1 quart capacity. The two gauges measure differential
pressure across the oil cooler bypass. The first gauge tells you the output from the relief valve; the second
reads 7-8 psi less if the bypass is functioning correctly. The accessory case in the photo is gray, and it is
just test fit in place. It does not have any of its systems installed.
Above is the 180 degree view. The output from the oil system travels down one gallery only, the other is
plugged. It exits the case and goes through a ball valve. This valve is adjusted to mimic the same amount
of oil restriction offered by an assembled engine. This will take a little calibrating, but when complete,
will offer us confirmation that the pump is flowing enough volume to feed the oil requirements of the engine.
From the valve, the oil is returned to the case through the aluminum line. The aluminum plug in the old hole
in the case is actually the top of a cast piston welded in place. Little goes to waste in our shop. The chrome
dipstick is a nice touch. The tool is c-clamped to the steel workbench for stability. Again, we've performed this operation countless times with the
accessory cover installed on an engine. The tool just expands our ability to perform this operation, either
before we build the engine, or for a customer in a remote location.
Here is a photo of a grimy rear accessory case removed from a core motor. Surprisingly, the great majority of them can be brought up to aircraft standards with careful work. 1964-1969 engines had die cast rear covers, which are far lighter than the 1960 to 1963 sand cast models. Although they are interchangeable, we only work with the light weight die cast ones. Since we only work with 1964-69 engines, it is rare that we see a sand cast model. The spring at the top is the mechanical fuel pump drive rod. This rod and its iron guide are not used in flight engines. Aircraft which use fuel pumps, like our 601XL, are better served by redundant electrical pumps. I know this from the experience of flying the plane both ways. We remove the guide because it is in the way of our new Oil Top Cover.
Here is the original tool we use to remove the fuel pump guide. It is a threaded rod and a tube for the guide to be drawn into. At the bottom left is a small threaded nut which goes on the rod below the guide to draw it out. The rod is 3/8”-16 thread, the tube is ¾x .058”. Those with sharp eyes will see that the washers are actually Corvair valve spring retainers. This tool can be used to pull the guide out of a fully assembled engine. This is the hard way to do it, akin to building a ship in a bottle. The much better was is to remove the guide before rebuilding the rear cover and installing it on the engine.
Above is the rear cover in our press. This is the easy way to remove the guide. The greenish shaft is a driver made from a 1/2” steel rod. Here it is pushing the guide out into a steel tube which is supporting the case. We push these out cold and have never cracked a case. It takes about 1,000 pounds of force, a very low effort in our 20 ton press.
Above is a cleaned up case. To get it to this point, it has been run through our Environmental Cleaner (the same type of machine currently used by many transmission shops), and then put through our Scat blast cabinet filled with walnut shells. Under no circumstances should this part be cleaned with any type of abrasive media like glass beads or sand. At the very least they would harm the oil cooler bypass valve, and possibly hide in the cover until the motor is reassembled and run. Don’t let anyone talk you into this. If you go visit a Repair Station which is licensed by the FAA to work on certified engines, you will never see abrasive media used anywhere it could possibly enter the oil system. Since we are building airplane engines, I use the aircraft industry as the standard, not the automotive world.
Above is the cleaned up rear cover, jigged up in the mill. The Smart level is on the area where the stock oil filter housing goes. This is the surface from which the guide was protruding. Here we are going to use the cutter to take a few thousands of an inch off this surface to clean it up and give the gasket a perfect mating surface. Keep in mind that when the engine is in operation, more than half of this area is subject to engine oil pressure, on gasket surfaces 3/16” wide. You do not want to tolerate any type of questionable surface. When complete, we use this surface to bolt our new Oil Top Cover onto.
Here we’re pointing to the oil pressure relief valve area. When building an engine, we boost the relief oil pressure slightly, from a stock pressure of 37-38 psi to 42-44 psi. Builders should note that I only do this for engines that will be using one of our ignition systems. While you first may wonder what they have to do with each other, remember that the ignition system’s distributor shaft runs the oil pump. The distributor drive gear is bonded onto the shaft to allow it to take the greater load of driving the pump to a higher pressure. This is a good example of how the work we do on engines is part of an integrated system which we designed to work together. The traditional way to raise the oil pressure in a Corvair is to exchange the relief spring for a hot rod one. The problem is that most of the aftermarket springs will raise the pressure to 55 psi or more. Old hot rodders used to stretch stock springs until they yielded, but this isn’t a good idea in an airplane engine. We get far more consistent results but milling the surface where the plug which holds the spring seats. By machining this down, slightly more compression is exerted on the stock spring, and the 42-44psi is achieved.
After the machine work is done, the cover is thoroughly cleaned, and prepped for painting. This cover was done by customers Vince and Louis working in our shop on their engine, which has a yellow/black theme. All the regular ones we do are our standard royal blue color.
Above, the hex plug is the installed oil pressure relief plug. The black cover is the oil pump. We carefully true the surface with 600 grit paper, and install a matched set of oil pump gears. The cover is carefully set for absolute minimum clearance. This gives the highest idling oil pressure. Typically, we can get 25psi at 900 rpm with hot, thin oil. If the pump is shimmed loose, the same engine might only make 15 psi at idle. I have a very large collection of gaskets which allow me to set up any pump with the ideal clearance. In this operation, I continuously check the effort required to turn the pump to make sure it has no spots which bind. When it is correct, the final gasket is installed with a thin film of Permatex spray copper gasket sealer (do not use imitations). Then, stainless Allen hardware is installed and safety wired.
Above is an overview of the rear cover with the Oil Top Cover installed. The rear seal is yet to be installed. When we ship these, we send them with instructions and stainless hardware to complete the installation.
Here is the top view of the Oil Top Cover. It is made from a machined 6061-T6 plate. The AN-6 fittings are special ones from Earl’s which are not anodized, making it possible to weld them. This is the prototype. Production models have far smoother welds. All of our installations will use this plate to replace the stock oil filter housing. A firewall mounted remote oil filter housing like a Transdapt 1045 will allow the use of a wide variety of commonly available filters and allow the oil pressure and temperature to be taken by mechanical senders in the oil filter housing. Again, this is my recommendation based on flight testing every combination of oil systems, with consideration given to instrumentation. Another example of how all the subsystems need to be considered as a whole and how they will function in concert.
Above is the view of the inside of the Oil Top Cover. The slot is required to allow the oil cooler relief valve to function when the oil is cold and thick. Without the slot there is not a clear path for the oil to address the face of the relief valve. Notice that the bottom gasket surface is machined as a last operation.
Here is a view of the complete rear cover for Vince and Louis’ 601 engine. This cover has the rear seal in place. We bond them in with RTV. Remember that the inner lip of the seal should be filled with lithium grease to protect the seal on initial startup. This is covered in the GM shop manual. The hardware is stainless Allen screws.
Here is the same accessory case on the test rig. It is being driven by the ½” drill. The drill is turning a distributor shaft in a stock housing. Everything in our shop which is painted orange is a tool or a junk part. (Any flawed part is either thrown out or marked with orange paint immediately to prevent it from getting into the production line.) The braided line is there so the oil system has a complete circuit. When installed on your plane, this one line will be replaced by the out and back lines to your remote oil filter housing.
When the drill is run, the first test shows that the pump will prime and draw oil from the sump. When pumping, the whole system is being leak tested. Pressure comes up on the two gauges quickly. The return valve on the test rig is set wide open to verify the volume of oil pumped at idle. The valve is then set to a restrictive setting to mimic the engine’s resistance to oil flow. The idling oil pressure should be around 25psi. The drill speed is increased until the oil reaches its relief pressure, which should be 42-44psi. Properly shimmed pumps will hit this at a fairly low rpm. When the relief is open, the second gauge should read the differential pressure required to open the oil cooler bypass, about 8psi. Much more than this, and the engine will starve for oil when it is idling cold. Much less than this, and the engine will have high oil temps in cruise operation because it will bypass the cooler in normal operation. Finally, the test rig valve is closed all the way and the drill is run up to full pressure and quickly stopped. If the cooler bypass is properly seating, the pressure on the second gauge will decay very slowly. At first glance it seems like a very simple tool, but once you spend some time with it, this tool can provide a lot of very good information on the oil system. When a rear case assembly passes with flying colors, we seal it in a bag until we put it on an engine or ship it to a customer.
If you would like us to overhaul and convert your rear accessory case, equip it with one of our new Oil Top Covers, and
return it to you fully tested, follow these simple steps:
Box up your greasy rear cover, including the oil pump cover and pressure relief, and ship it by UPS or FedEx to our hangar
at 735 Air Park Road, Hangar A-3, Edgewater, FL 32132.
Include a check payable to William Wynne for $249 for domestic orders
by Jan. 30.
Or click here to pay by credit card via PayPal:
We'll cover return S&H. This is our introductory price through Jan. 30, 2005. By choosing an Overhauled Accessory Case with our Top
Cover, which has 50+ hours of proven flight time on it and is made of the best materials, you have taken a giant step toward
minimizing the risks associated with oil systems. If you have any questions, please e-mail us.