Aluminum push rod tube issue resolution.

Builders;

This commentary ONLY applies to aftermarket aluminum push rod tubes, which are  in maybe 3% – 5% of Corvair flight engines. If you have stock steel push rod tubes like the powder coated ones we sell, it does not apply.  However, the notes still make good reading and give some appreciation for detail design considerations GM put in the engine, and unforeseen potential consequences in modifications, particularly in combinations with other work.

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The original design of the aluminum tubes goes back more than 15 years. They were the work of Charlie Johnson, aka “One Sky Dog”. I’m pretty tight with Charlie, you can see pictures of the two of us hanging out at my place in Florida, building engines and shooting AR’s in this 2017 story: Last Engine of CC #39

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Part of the awareness of this issue with the aluminum tubes surfaced when we were building and testing Charlie’s personal 3.0L for an upgrade in his flying Dragonfly, read: Corvair powered Dragonfly, Charlie Johnson, aka ‘One Sky Dog’ .  The basic issue is this: When the aluminum push rod tubes are used in an engine with heads milled for a tight quench area, they restrict the oil flow back to the pan, and oil builds up in the valve covers.

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This isn’t a small consideration, I have seen a motor with 4 quarts of oil in it suck air into the pick up because it had filled each valve cover with 2 quarts, and it was in the process of ejecting a lot of it out the breather tube.  This wasn’t an easy issue to spot, because it is dynamic, and if you stop the engine and wait a few minutes, all the oil shows back up in the pan. Sight tubes on the valve covers confirmed the issue and the relatively simple solution.

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Above, is my own personal 3,000cc Corvair, (3,000cc Corvair (lower compression) engine), and you are looking at the #1 cylinder with the intake rocker removed. Notice there is  no space between the guide plate and the push rod tube. that is the normal path oil used to flow back to the pan. In operation, a lot of oil is pumped through the lifters, up the hollow pushrods, and sprayed out sprinkler style onto the rocker and the valve spring to cool them. It drains back by gravity through the push rod tubes, but it can’t be restricted. The stock steel tubes have never had an issue with this, but the following pictures will show why it is an issue with aluminum pushrod tubes.

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Above, same part of the engine from a slightly different angle.  The tube leaning on the head is a stock GM steel push rod tube which has been powder coated. Notice the wall thickness at the top is only .035″. This allows plenty of room for oil to flow between the guide plate and the tube. Now look at the Aluminum push rod tube in the engine. It’s wall thickness is more than 3/16″, .187″ This can block the return oil flow. The hole in the guide plate is where the pushrod goes, and in operation the pushrod is moving several thousand strokes a minute, oil doesn’t return through the space around the pushrod.

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PS: The photo shows a roller rocker, but I just removed these from the engine. I have a few hundred hours on them since 2004, but I opted to replace them with new, standard Elgin made in USA rockers as a pure reliability issue.

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Above, Why some Corvairs have used aluminum pushrod tubes without this being an issue, and why it is a problem in others: This is a very Common 3.0L Corvair set up. The “step” in the head, an upraised area where the head gasket sits, has been intentionally milled down to get a better “Quench clearance”. This in combination with any of our 2,775, 2,850, 3.0L and 3.3L engine kits with special designed dished pistons produces a very detonation resistant engine with a very desirable tight quench / moderate static compression ratio combination. We have built motors this way for almost a decade, but never encouraged the use of anything but GM steel pushrod tubes in them. Thus it was a long time before I saw the rare example of the dysfunctional combination. As luck would have it, my own personal engine had the combination, so I got to test and address the issue for other builders. My own engine has tested a lot of stuff, and in its details isn’t typical of engines we teach role to build for themselves.  BTW, we did nearly the same modification to Charlie’s personal engine which has aluminum tubes also.

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Above, on the left is the stock aluminum tube as they are made. I had these powder coated, like we do to steel GM ones.  On the right, the same type of tube which I just fed through my lathe. I shortened the tube .125″ and spend the end of the tube to give a generous oil return clearance around the guide plate when installed.

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Above, I was trying two different end treatments, but either one will work to resolve the issue.

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Above, proof of anything requires testing. Above is the engine with the modified aluminum push rod tubes running on my stand in front of my house.  Its green in the picture, but it wasn’t real warm. This issue shows up mostly on warm ups in cooler conditions, but if your engine has aluminum push rod tubes in it, this really needs to be checked and addressed.  Again, this doesn’t apply to steel GM pushrod tubes.

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15 or so years ago, when Charlie made the first set for himself, these rapidly because a hot discussion topic on the internet, particularly on the Corvaircraft list. People who had never held one, far less put one in a running engine, spent a lot of time talking about how great they would be because the set weighs 15 ounces less than the stock ones, and they must cool the oil.  I spent some time debunking the myth of cooling, as the oil in the head is at least 150F cooler than the top of the tube in operation. It meant little, as the discussion was dominated by people without a running engine who were sure their opinions were more valid than any measurement I took.

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For the record, Charle has not made any tubes in many years. He was never the guy driving the internet claims about them. Soon after he made a few sets, a west coast Corvair Car company made copies of them, and he drifted away from the project. In the end, this isn’t a tough issue to fix, but it is a lot more work than meets the eye because the heads have to be un-torqued to remove them, and without a lathe, it would be hard to fix. In the end it is a bit of a precautionary tale of how a small combination of alterations can produce things like the oil pump sucking air in an engine with plenty of oil in it. In the end, the great majority of builders are best served by very specific combinations of parts that we use every week to build very proven engines.

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Wewjr.

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PS; Something to laugh about: In the story: 2018 Zenith Open House -The long run., I pointed out the presence of a salesman, who was promoting an engine which had never flown on a Zenith, which he didn’t own, nor had ever even seen one run. For a moment, contrast that with the type of detail and testing above, and consider the 29 years I have been doing this and the 1,100+ stories on this site alone, and all the builds and engine runs, and colleges, and ask yourself what kind of person would listen to a salesman who has never even heard his Chinese engine run.

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About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at www.FlyCorvair.com and in more than 50 magazine articles.

5 Responses to Aluminum push rod tube issue resolution.

  1. David says:

    I have a question William, what is the advantage of using these tubes other than the small weight savings? If any? Thanks

    • David, their big appeal was they looked nicer than most sets of steel tubes, and people started to imagine advantages like ‘cooling’ to justify simply wanting something that looked nice

      • Charlie says:

        William and David, Yes Pat P. and I were thinking of weight savings. I also have a (1) molded 350F epoxy carbon fiber valve cover that will fit over roller rockers. I went to a tooling class and had to have a project so why not? It is really light, but then I saw the light and wisdom of following the progression of you and Dan. I had a really old manual pre 2000 and followed it pretty close. For the upgrade to the 3000cc engine I am using all of the Gold parts and my aluminum pushrod tubes. William thank you for getting the word out and publishing a great modification.

  2. David says:

    follow up question, is there any advantages to using roller rockers?

    • None that I have ever discerned, and there is some question if the geometry is actually close enough after the valve guides are replaced. Standard rockers don’t have this issue.

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