Custom Motor Mount Questions

Our Friend Rob Schaum, who is building a Murphy Rebel to be powered by a large Corvair, wrote us this note with a number of welding questions related to his quest to build his own motor mount. Rob bought one of our Motor Mount Trays and Spool Sets to get started, and then did some very impressive motor mount calculations that he ran past me. (His work turned out to be very well organized and accurate, best I have seen from a homebuilder.) His questions are far outside the simple scope of building a Corvair engine, but the engine by itself isn’t going to fly your plane. Unlike many people who market engines, I actually know how to mount them on planes.  Over the years, I have seen a number of companies say things like “liability prevents us from commenting on that.” In many cases this is a face saving way of dealing with the fact that they sell imported engines, but don’t know any of the details of how you would do a custom installation. I have built more than 50 different Motor Mount designs for the Corvair, and I stand ready to help anyone with a question they may have with their installation, even if it is a one of a kind.

Hi William,

So I’m ready to start fitting tubes, as the motor mount jig is all ready to go and firmly attached to the work bench (see pic).  

In preparation, I have read Finch’s book, and also L.S. Elzea’s WW2-era bible on aircraft welding.  I am now all set up to start practicing on the “problems” at the back of the book (essentially practice exercises).  I have to say, it’s a great book, and the “problems” do a great job describing techniques specific to specific steel tube structures and configurations.   I expect that the most challenging parts to this will be joining the tubes to the heavy spools, due to the thicker spools sucking up all the heat.  Also the 2-tube cluster (see above) might be challenging.  I have some spool “stock” on hand to practice those specific welds, and plan to practice the exact cluster a few times before doing the real thing.   Have a look at my set-up in the picture and let me know if there are obvious flaws.  It is actually really sturdy front-to-back and laterally, but I am most suspicious of the twisting loads created while tacking-on the tubes. Some questions I had on the actual welding of the motor mount:

1) I can’t find any closeup photos of your 2-tube clusters at the lower firewall mount points.  It definitely looks like the short tube is fitted first, followed by the long one, saddled primarily on the first tube due to the acute angle with the spool.  However, I can’t determine whether the shorter tube is completely finish-welded before welding-on the longer tube, or if the shorter tube is tack-welded first, followed by the long one, then the cluster is welded as a unit (the latter appears to be standard practice in the literature).  Can you please describe this procedure/area to me? 


When you weld a cluster, it is not required to weld the parts of the cluster that are covered by the tubes placed later in the cluster. Basically, you are just welding the seams of the cluster that are visible on the outside of the completed joint. If you think about it, the forces on the joint are going to be transmitted through the outer surfaces of the tubes, and the welds that would be hidden inside would not be doing much work. I have cut apart a lot of welds in certified planes that have been around for decades, and none of the planes had the interior layers of the tubing clusters welded, even in the motor mounts or landing gears. Your assessment of the order of placing the tubes in the jig is correct.


2)  Your motor mount 101 writings also refer to finger-straps on the 2 top tubes…do you still advocate that?  I like the extra insurance at this location.  Seems like one should have them on both ends (spool and tray).

Yes, they are a good idea.  If you look at the November update on our website at you can see photos of our personal Tailwind mount, and if you look closely, you will see that I put the tabs in where the upper tubes contact the tray. In our case, you have to remember that a Tailwind’s mount also has very high gear loads going through it. If you are an amateur gas welder, it can’t hurt to put the reinforcements in as outlined in our Conversion Manual ( available at the link). They are there to absorb tension loads on that joint. We do not use them on Mounts like our Zenith designs because I can get the full strength of the tube out of the joint by having it wrap slightly around the Tray at the contact point, and using 30 years of welding experience and a top of the line TIG welder to make our production Mounts. If this is your first mount, and you’re using gas, put them in. We don’t have them at the top as commonly because the top joint to the spools wraps around the spool, putting a lot more of the weld bead in shear, which is much less failure prone than a straight tension weld.


3)  Is there an overall sequence to attaching the tubes to minimize distortions in the geometry due to weld-cooling stresses?  Tack weld everything then finish weld?  Or tack and finish each tube (or each matching set of tubes) then move on to the next set?

Tack weld everything then finish weld. To minimize distortion, work your way around the mount; it is good to do part of one cluster and then part of the next. There is no harm in this as long as you heat up and cool down the joint you’re welding each time. Something like 60 seconds leading in and 120 exiting in still air. Do not gas weld in a room that has air currents in it.


4) I had planned on tack-welding everything, then taking the thing off the wooden jig and test fitting on the plane.  If all looked well, I was going use the tacked mount to construct a steel jig (I’ve been studying photos of yours), and do all the finish welding on the completed steel jig to avoid distortions that might otherwise occur using the wooden jig. Obviously, it would also enable better positioning of the work for the finish welding process.  Is a tack-welded mount sufficiently strong to act as a “jig” for a jig, or will distortions generated during the construction of the jig end up “popping” tack welds on the mount itself?

If you are reasonably gentle with it, it will be fine. Try to put at least two tacks on each joint, but three is better. Try to space the tacks around the joint so they are not bunched up on one side. I would resist trying to gas weld a jig. It it is made out of strong enough material, it will be hard to get enough heat into it for welding without distorting the structure though warpage. If you have a buddy with a Mig or stick welder, burn the jig together using one of these techniques; they produce instantaneous heat which keeps distortion in check. Consider bolting your jig together. If you do weld it, pulling the tacked mount off it and checking it on the plane again isn’t a big step.


5) If I screw anything up, how structurally sound is it to cut the offending tubes off the tray, grind flat, and start over?

A lot of books act like this is a big deal, but it isn’t. If you couldn’t do it, then how would repairs be accomplished on steel tube planes? If you don’t like something, just cut the tube out, grind the weld bead away, and start again. The main thing that you want to avoid doing is running the flame set to an oxidizing flame (too much O2); this will BBQ the steel and it will take on a slightly rough, baked texture. If you keep going over a weld area with a flame like this, you are harming the base metal.  Use good sense, and if you don’t like the way things are going, stop, take pictures and send them to me and we will figure it out.

6) Time permitting, are there useful weldments I should be attaching, or are Adel clamps the norm?  What about attach points for the SS 1/8″ safety cable?

A safety cable can be threaded through the mount and bolted back onto itself, I would not weld tabs on for it. I would weld a battery ground cable strap onto the right rear corner of the tray; this will go to the back of the right hand cylinder head. Most of the other stuff will use adel clamps.

7) Much has been written about ambient temperatures for welding.  How strict must one be in maintaining the ambient around 70 degrees, or can careful withdrawal of the flame compensate for virtually any environment?  My garage is unheated, and it is currently 24 degrees F right now….perhaps I should do the finish welding in the basement?

In 1981 I was rabid about motorcycle drag racing. I lived in New Jersey, a state that regards drag racing as a birth right and a modern form of dueling. Englishtown was only 22 miles from my house, but like most young guys we were drawn to the “you can’t break the rules, we don’t have any” attitude of Atco, a track that was sanctioned by IDBA, the non-family entertainment version of the NHRA. That winter, my friend Ben and I welded up a new frame for our 830cc Kawasaki H-2. It had an all out Denco engine and in a fairly stock chassis had run 11.22 in the quarter. We were hoping that a new frame and an air shifter would get us in the 10’s at 120+mph. ( If you ride a 1,000cc Japanese sport bike that may not sound quick, but we are talking about an era where bikes handled like shopping carts with a bad wheel and  a $29 Avon Speed Master II was considered a great tire because they usually stayed in the front rim when they went flat.) We welded up the frame using a gas torch in a 30 degree garage. When it was all done it looked as stout as the Pulaski Skyway. A dopey friend asked how we knew it wouldn’t break. I considered the question a serious insult. To demonstrate how strong it was I picked it up to chest height and dropped it on the floor in front of him. I was stunned that two of the welds had cracked!  It was ugly to think about what might have happened if they had popped in the top end of the first run. No matter how you’re dressed, no one wants to think about how far you will slide at 100 mph on pavement.

Do not weld anything in a shop that is below 70F if you are building your own stuff for the first time. Pros can stretch this to a much cooler number, but it is a very bad idea to try to get away with this in your first go around. Find a warm spot and stack the deck in your favor.

The steel jig would go to you in the hope you’ll be able to save the next Rebelvair builder some time. 

That is the kind of thinking that I have always found to be the best thing about the Corvair movement. I am glad to take the time to help any builder learn something, but it is especially rewarding when I can tell that the guy is already thinking of other builders who will follow him. Most other things you can do in aviation don’t have very much of this element anymore. It is unfortunate, but I recognize that I can’t change the commercial direction of aviation. The good thing is that I don’t need to, I am happy to just make our corner of Corvair power an oasis where builders who are here to learn, create and have fun have a place to be among friends.

Your help/wisdom here would be greatly appreciated.

Thanks, Rob

You’re welcome, keep us posted on the progress.

Have a good night,




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