Jerry,

I had this idea quite some time ago but have not had
time to knock it out in Rhino. I think you
will find this much easier to fab than my original idea
of the aluminum spacers and rolled 3/16 sheet aluminum
tig welded to the spacers. You can weld this up anyway
you wish. It uses 1/2" 4130 .035 wall tubing bent using
a plywood form. The radius is about 7 inches. Heat the tubing,
clamp one end and pull it around the plywood form.
Not all diagonals will be needed to be bent to clear the
starter ring gear.

I have never been entirely comfortable with the aluminum
block configuration for several reasons. High weight among
them. I like this idea a lot better.

Paul Lamar

Paul, is  there anything to be gained by completing the circle across
the bottom?  Also,  is  is permissible  to form the right hand
diagonal to clear the starter?    Jerry

I think the bottom is best done using this idea. I don't consider
it necessary but it makes certain builders more comfortable.
This too should be pocketed out if you can afford it as making it
out of solid plate is rather inefficient use of material.
I suppose I could do a casting if we achieved some sort of
unanimity :)

I think the bulge in the starter pinion holder would cause too much
of a bend in the tube. I could be wrong. You might want to mock it
up and take a picture. Bent tubes are not ideal compression and
tension resisting devices. Only the local bending stiffness of the
tube shape makes them marginally effective when used in space frames.
It is best to keep all tubes as straight as possible.

Of course is it widely done in successful Lycoming and Continental 
motor mounts and for roll cages in racing cars. Any time something 
like this is designed it should be inspected for cracks on a regular 
basis until years of experience have been accumulated to insure there 
are no problems. This in fact is what has been achieved by the current 
crop of certified aircraft bent tube engine motor mounts. Lacking that 
a computer FEA of the proposed bent tube motor mount design could be 
done to gain confidence.

Paul Lamar
 
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