engine shock mount SDR

[SteveG]

Kim, I appreciate your taking the time to consider this situation and for all that you
contribute to this forum. A well reasoned argument is never snarky and would never be
taken as such. We are both trying to achieve the same ends; to realize a shock mount
installation that is in conformance with its design intentions and is performing optimally.
However, please indulge my continued skepticism the reasons for which I shall endeavor
to set forth as follows:

Firstly, yes, if the mount halves were to make contact with one another grossly in
advance of contacting the engine mount spacer it is conceivable that the assembly could
fail to achieve unity however these parts are designed and manufactured to close
tolerances. The outside diameter of the mount flanges are just a sliding fit within the
inside diameter of the engine mount spacer. When the assembly is drawn together using a
7/16” diameter fastener tightened to 40 lb-ft of torque a tremendous clamping force is
achieved unifying the component parts in tension along their mating surfaces. They’re
not going anywhere.

Secondly, I have attached hereto Lord Engineering drawing no. S-6493
detailing the design installation configuration for the J-9613-31 mounting kit and its
component parts. I would draw your attention to the cross-sectional view in the upper right
hand corner which depicts a single mating line through the forward and reverse C’s of the
metallic shoulder and flange of the mount halves. There is no double line indicating the
intention of a gap of 0.15” or of any similar magnitude. Also, please note the dimension of
0.62” across the mouth of the C's. In other words a dimension of 0.31” as being the flange
height of each mount half. These values can be confirmed by measurement of the actual
part. In short, this manufacturers technical drawing conclusively depicts the installed shock
mount halves are intended to mate upon assembly.

Thirdly, with respect to achieving the proper preload compression using an AN7 bolt, this
is the issue which started this whole critical investigation. The internal length of the
mated, uncompressed mount halves minus the length of the internal spacer is 0.41”. This
dimension could be called the compression distance. When the mount halves are
separated by 0.15” this compression distance increases to 0.56”. The length of threads on
this bolt is somewhat variable depending a manufacturer but is generally between 0.5”
and 0.6”. On the new hardware purchased for this installation the thread length measured
approximately 0.5”. Subtracting 0.1” to allow the nut to be started on the first full turn of
thread leaves 0.4” to 0.5” of thread available to do useful work. This is marginally
sufficient to accomplish 0.41” of compression distance in the mated configuration but is
wholly inadequate to realize the 0.56” of compression distance which would be required
to achieve capturing the internal spacer in the gap configuration. And, if one had a
fastener of sufficient thread length to realize the mating of the upper and lower mount
halves with the internal spacer, the resulting compression of the mount would be some
37% greater than the design intention. This significantly stiffens the assembly ( degrades
its elasticity ) and compromises its ability to capture and isolate vibration passing into
it.

I find the forgoing compelling evidence to conclude that these mounts, as employed in
this application, fail to conform to their approved design criteria and are performing less
than optimally. If you have access to data which allows for a reasonable alternative
conclusion I would be most interested in considering same.

Sometimes the “facts” that we are most sure of are the same “facts” that contain the
greatest error.