>Yes, sure I've been aware engines usually are rubber-mounted on their
>mounts, and of course ground straps between engine block and mount have
>always been common practice.
>
>What I've been wondering about, however, was the remark that, as I
>understood it, the actual structure of the mount itself should never be used
>as ground lead. Is that true, or is it just a matter of not defining things
>precisely enough?
The advise against using engine mounts for ground paths
is based on several relatively minor points. First, the
mount is generally tubular steel, not nearly the quality
of conductor that a copper ground strap. There are a myriad
of joints and attaching hardware involved in the grounding
path each of which contributes a small amount of resistance
in the milliohm or sub milliohm level. Engine mounts attach
to the airframe in generally 4 or more places which causes
currents from the mount to flow across the firewall sheet from
a variety of directions. Fasteners designed for adequate
mechanical performance may or may not be good also for
electrical performance . . . given that having an engine
fall off is more stressful than having poor electrical system
performance, efforts to optimize mechanical integrity
tend to prevail.
Any one of these considerations by itself is relatively
minor but if anyone has read my articles on grounding
will know that the sum total of these effects can add
up to significant performance losses and/or noises. The
obvious solution is let the engine mount hold the engine
on and let a ground strap carry the electrons to and from
a single ground point on the firewall. BOTH may be
optimized for their respective tasks without compromising
the other.
Bob . . .
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