Peter Cowan/Lexi Cameron wrote:

Paul, again this question comes out of my ignorance but... with traditional
mount design the equivalent firewall attach tube is totally retained by the
mount bolt. With your design the tube is retained by the circular weld at
the washer. Is this AOK?

The traditional design also relies on the weld between the tube and the
the small, usually 5/8 inch diameter, tube that the bolt goes through.
there is no room for a gusset. 

With this design and  a 1 inch tube the
weld length is then about 3.14 inches long. If the weld penetration is
is 0.065 inch wide you have a area of about 0.2. At a weld stress level
equal to normalized 4130 or about 60,000 psi this joint should take 
0.2 X 60,000 psi or about 12,000 pounds. Assuming for the moment we 
have roughly a 2,000 pound tensile load on each top joint during a 6 G 
pull up it looks like our factor of safety is about 6. Most of the time
the motor mount is subjected to one G plus torque and thrust loads.
Vibration of course is a factor so the motor mount should be checked
once and awhile for cracks. It is a good idea to normalize all
the welds.

I would use an oversize washer under the head of the bolt or nut as the
case might be inside this fitting. Not an AN washer. The shear area on 
the .065 round plate given a 3/4 inch washer would be .115 or about 6800 
pounds or a factor of safety of about 3. So the weld is probably stronger 
than the round plate. The bottom firewall joint is under compression so things are 
somewhat less critical. I may go up a hair on the thickness of the small round 
plate. Keep in mind of course most tubing used in motor mounts
welded to these parts is 3/4 inch or less in diameter and 0.049 
thick 4130.    

Paul Lamar
 
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