Jeff Cochran, CH-750 builder from Alabama with a running 2,850 cc engine, writes:
Welcome back to the world wide web. You have been missed. Questions about the installation of your SS exhaust pipes. First, if ceramic coating of mild steel is bad, what about wraps on the SS system (except for the heat muff section)? Next, do the Heat Muff Box Ends need to be attached to the pipe and if so what is the best and worst method? And last, you say the pipes do not require tail pipe brackets, but the 601 Installation Manual calls for a steel tubing brace across the ends? The new site is great, keep blogging.
Good to hear from you. The photo above is the first run of your engine at Corvair College #19.
Wrapping the pipes is bad for mild steel for the same reason why ceramic coating the outside of mild steel is bad: It keeps heat trapped in the steel, and mild steel can’t take this. If you look at the pictures of our Pietenpol in the late 90s at our http://www.flycorvair.com/carbice.html page, it had wrapped exhaust. I learned my lesson then. As a concept, it is worse than ceramic coating steel because when it cracks or disintegrates, you can’t see it. The only Corvair builder who I can think of who found this out the hard way was 601 builder and pilot Scott Laughlin. His wrapped mild steel exhaust gave in in about 100 hours, but he initially didn’t see it because it was wrapped. Wrapping the exhaust had its heyday in drag racing 25 years ago before coatings were available. Today they are a fashion statement on custom motorcycles. I can attest that it doesn’t work all that great either. My motorcycle, a Buell XB12X Ulysses came secondhand with a wrapped exhaust right where it passes my right thigh. It still radiates enough heat to be very uncomfortable. Sooner or later I am going to send the header pipes out to Jet Hott in Texas to have them ceramic coated inside and out. The best way to secure the heat muff ends it to get the box built and fitted right where you want it and then let a local welder put two tack welds on each end. The welds don’t have to be very big, two spots 1/4″ in diameter will do it. Other builders have used a hoseclamp above and below the box. Avoid anything that would puncture the main exhaust tube like a rivet or a screw. Your Zenith Installation Manual is an early one where we experimented with tying the ends of the pipes together aft of the nose gear. Subsequent experience has shown that this isn’t necessary.
This follow up came in from Gary Burdett, 750 builder from Illinois, also building up a 2,850 cc engine:
I take it that the short stubs are the place for the egt clamps.
If you’re planning on 6 egts, the stacks are the place to go. However, a majority of Zenith builders are using just 2 egts, one in each pipe, allowing them to monitor each side of the engine. In this case, they mount it about 6″ past the last stack.
Above are two of our powder coated modified Valve Covers. On Tuesday we are taking another batch to the powder coating shop in Jacksonville. We will have more sets of red, and the new blue will match the Old Ford Blue that we have traditionally used on FlyCorvair.com stuff. The above blue is known as Royal Blue, and we have one set left of these. We are getting several more sets in black, and on Tuesday I will try to get any custom color that builders like, if they order the covers this weekend. The Valve Covers come as a pair, the other side has the oil breather line fittings. You can have your choice of Valve Cover stickers: 100 HP, 2,850 cc, 120 HP or FlyCorvair.com, just please let us know with an e-mail to WilliamTCA@aol.com. If you would like more information, read the November 2011 Hangar Update page of our FlyCorvair.com Web site: http://www.flycorvair.com/hangar1111.html
Helicoil vs. time serts: Had to remove the top case studs due to completely corroded studs. Helicoil is what you recommend. What about time serts?
Thanks for the question. In the photo below, you have time sert tools on the extreme left, and the other three packages are popular variations on the helicoil concept.
I have used all of them over the years. They will all work, and any of them will hold in the case the full strength of the stud, if the stud is installed with Loctite 620 upon assembly. Given a preference, I would pick a time sert, but it is just a style point, not a major issue. The time sert stuff is a lot more expensive than helicoils. If you look closely, the thread on the Recoil box says 5/16-18; this is for other places on the engine, I just had it handy for the photo I shot a few minutes ago. The thread for case studs is 3/8″-16. This type of repair is something that we demonstrate at every College. They are not difficult to do if an experienced guy shows you once. I have put literally several hundred of these in Corvairs that have gone on to fly for many years. My own personal engine has about 30 of them in it. When done correctly they are stronger than the original threads.
We received this from Davis DA-2A builder Carl Luhning in Canada:
A recent comment regarding the a Harmonic Balancer coming loose leads me to wonder if I have installed mine correctly.
I used the called for Loctite for shafts and tightened it to the proper torque using the correct bolt for the purpose. I did not however use Loctite on the threads of the bold. Should I have and if so which one.
Thank you for your reply.
Good to hear from you. Follow through these photos and descriptions, and you can verify that your balancer is correctly installed. I would like to emphasize that having a properly installed balancer come loose is unheard of in the land of Corvairs. GM made 1.7 million Corvairs, and about 600,000 of them left the factory with a balancer installed. Over a lot of years, the original rubber elastomer could rot, and allow the outer inertia ring to slip or wobble. If you read stories from car guys, this is the type of problem they are talking about. This is why we tell people to get a rebuilt or new balancer with modern synthetic elastomer. But the point is that the hub itself, where it fits to the crank, is not a common issue that builders need be overly concerned about, with 2 exceptions that I will bring up further down.
The abovephoto shows the stock washer and bolt that hold the balancer on a Corvair. The bolt is a grade five 1/2″ -20. The washer is a very special part made by GM. It is made of heat treated steel, and it has a very slight concave surface on the side that touches the balancer. When it is tightened, this washer goes flat, and spring loads the bolt tight. This is why in its stock form on the car, it doesn’t have a lock washer nor Loctite applied to it. The “bevel” washer is doing the locking task. On cars, this system works flawlessly. On aircraft, it works great also. However, it is important to note that some people who choose to use a rear starter assembly do not have this washer incorporated in the system. Our Pietenpol had a rear starter from 1996-2001. It had an aluminum Puck that held the ring gear onto the balancer. This Puck had a pocket on the rear face of it so that I could use the same washer with a longer bolt. In 2009, we built several rear alternator set ups for engines that had our standard front starters. These had a Puck like part that also had the v-belt pulley. On these systems, I made sure that we had a provision to safety wire the mounting bolts in lieu of being able to use this washer. If a builder uses our standard front starter/front alternator, he just used the GM washer and bolt on his balancer, and he will have the same reliability that the cars have experienced.
The above photo shows that the GM washer is nearly 5/16″ thick. It takes this thickness to do the job of only going flat when the full torque is applied to the mounting bolt.
Above is a photo showing where the washer fits in the balancer. The balancer shown is a rebuilt DaleMfg.com unit.
Above is the bolt and washer going into its pocket on the back of the balancer. When I install a balancer I do not use Loctite on the threads of the bolt. I use oil on them. This will not affect the ability of the bolt to stay put, because the special washer is doing the job, not friction on the threads. Rebuilt balancers come with a special tube of Loctite that goes on the inside surface of the balancer, where it would contact the crankshaft itself. This takes up any space that may have developed over years of use. There are also new balancers sold by Clark’s that have a precision fit, but I would still consider using the Loctite because the crank itself may have experienced slight wear having the balancer removed and replaced in the past.
When the balancer is correctly installed on the engine, there is a gap of .150″ or so between the balancer and the pad with the timing marks cast in it.
Above, the part in my hand is the oil slinger that goes on the back of the crank, aft of the brass timing gear. This part is installed before the rear case goes on the engine. It is loosely floating on the rear of the crank until the balancer is installed. When the balancer is mounted all the way, this part is firmly clamped down. After the balancer is installed, this part can be seen by looking down the distributor hole.
Above, the last step in the process is to reach your finger down the distributor hole and touch the oil slinger. If the balancer is down all the way, it will be tightly clamped in place. If the balancer isn’t all the way down, even slightly, the slinger will have a detectable wobble in it.
Above is the other issue about balancers. If you look closely, the cross sectional area between the keyway and the outside seal surface of the balancer is only .125″ (1/8″). On cars, this has never been an issue. However, on aircraft there are two things that can cause a problem with this. First is a prop strike. If your engine experiences a prop strike, this keyway is going to get stressed. The balancer has a fairly high rotating moment of inertia. In laymens’ terms, it wants to keep rotating, and the fit between it and the crank, and the shear value of the woodruff key is all that is stopping it. Experience has shown me that in an overload between the crank, the key and the slot in the balancer, the loser is most likely to be the balancer cracking through the slot.
The second thing that some planes have that cars don’t is rear starters. The rear starter ring gear weighs less than the 5 pound balancer, but it is larger in diameter. It roughly doubles the rotating moment of inertia. In a prop strike, it goes after the keyway savagely. At Corvair College #13 in California, Pat Panzera brought out a rear starter engine that he purchased from a guy online. It happened to come from a builder I had visited in Florida, right after he had a prop strike ground running his Baby Ace without the tail tied down and without the wings on it. When Pat brought the engine to the College to run it, the engine showed very poor starter engagement. At first glance someone thought the crank was broken at the back, but upon a little digging the issue showed that the keyway on the balancer had split to the seal surface, allowing the balancer to become loose and the starter ring gear mounted to it to wobble.
In the above photo I have a hub from a GM balancer that I hit with a 24 ounce ball peen hammer about 5 times. What I would like people to look at it is the surface of the material where it broke. The photo isn’t great, but the material is clearly porous. It is cast material. It you put it on a grinder, it throws yellow-orange sparks, indicating that it is some sort of cast steel. (Cast iron on a grinder throws a more reddish spark.) Material like this works, but it isn’t as strong as if it were a billet part or a forging. GM engineers knew what they were doing and there is nothing wrong with this part for the task it was designed to do. The only issue that comes into play is when this type of cast material in a section only 1/8″ thick is subjected to an excessive load asking the part to absorb a shock load, be part of the starter system, or both. Rear starters work, but when they experience an overload, or if the mounting hardware is not safetied with the stock washer or wire tied, the balancer keyway stands a good chance of being damaged. This is the primary reason why a balancer on a plane would ever come loose. Excluding the issue of prop strikes, there is no issue for a person with a Front Starter to be concerned about.
Rear starters have been used for years, and our Pietenpol flew several hundred hours with one. They work. However, basic observation shows that they work the keyway much harder than a standard front starter installation. We are not just speaking of prop strikes. All the force of starting the engine is transmitted through the crank to balancer fit and the keyway. If the timing isn’t set correctly or the engine kicks back on cranking, there is a tremendous amount of force going through this area. If you have some doubt about how strong a starter is, ask any mechanic if they are powerful enough to bend a connecting rod in an engine that is liquid locked or has a piston stopped by mechanical interference. The three Corvair flight engines that I have seen that have broken a balancer through the keyway have all been rear starters.
This is not an across the board condemnation of rear starters. It is just a frank discussion to remind builders using them that they have things to stay observant of. It is also to remind all builders that when people discuss an issue, it may or may not apply to your aircraft. Today, more than 90% of the flying Corvairs use front starters. These engines have a good track record just like cars because they use the balancer and its hardware just as it is used in the car. Rear starter installations have different hardware, more weight, starter forces and a higher moment of inertia to contend with.
Over the years a number of Corvair powered aircraft that began flying with rear starters have changed over to our Front Starters. Two examples that come to mind are the KR-2s of Steve Makish and Bob Lester. These planes began flying on Corvair power is 2000 and 2001 respectively. They both originally had rear starters. Over the years, these two guys tested a lot of different ideas, some good, some not so good. They were open minded about it, and went with anything that caught their interest. The two of them have a motto, “The pioneers take the arrows”, meaning that if they were the first guy trying something, they knew that it may or may not work. Over a lot of years, these guys helped to define the reliable Corvair that we have today. In recent years their priorities have changed somewhat, and they have both distilled their own Corvair powered KRs to a configuration that they expect regular reliable service from. Today both planes are flying on 2,700 cc engines with Weseman bearings and our Front Starter System, seen here http://www.flycorvair.com/starterkit.html on FlyCorvair.com. You can check out our Corvair College #17 coverage at http://www.flycorvair.com/cc17.html and see pictures of each of their modern engines running for the first time on my run stand. If you’re a builder working today and would like to benefit from the decade of experience and 800 hours these guys have, consider building an engine that replicates the starter these guys are flying today.
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