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Eng. mount to airframe bond

Helmetfire

FRIEND
Caldwell, Texas
How does one insure a good bond between the engine mount and the airframe??

I was installing a freshly painted engine mount to the airplane and in the back of my head I was figuring on the bonding strap between the engine and mount would take care of any grounding issues. Then it dawned on me that a good bond between the engine and mount doesn't necessarily mean the mount is bonded to the airframe...

I painted up the lugs that attach to the firewall and also the mount itself and used new hardware throughout. I guess its possible there is some bare metal contact somewhere in the four attach points that would suffice for a good bond. I'd rather not scrape to bare metal unnecessarily so I ask you guys, what do you do??

Thanks
 
use a #8awg wire from one of the 1/4" studs on engine to one of the #8-32 screw studs coming through firewall..

that will provide a good ground while being flexible... its a short distance so you do not need a large wire(like some idiots are about to post)
 
I had to replace the engine mount on my C180 last year due to the original being cracked.
Had the replacement mount epoxy primed, with white polyurethane over that to make it easier to spot any future cracks.
Unfortunately I realized later that I hadn't scraped the paint off the mount where it attaches to the firewall. Dammit!
The starter seemed to crank just fine, which would be the biggest, highest amperage problem, but I still wasn't comfortable with it
so I added a ground wire from the engine to the firewall as Mike MCS suggests. As I recall I attached it to the studs for the vacuum pump mounting pad.
An electrician buddy looked up the amperage rating to determine the required wire size, it was a lot smaller than I would have thought.
 
use a #8awg wire from one of the 1/4" studs on engine to one of the #8-32 screw studs coming through firewall..

that will provide a good ground while being flexible... its a short distance so you do not need a large wire(like some idiots are about to post)

I take no offense to that!

I'm electrically not the most experienced so I put a 2 gauge braided cable from the case to the firewall, then another from the other side of the firewall to a frame ground.

I'll admit that it's ridiculously overkill. I should have asked the brain trust before I started tinkering.
 
What Mike said.

It's the most secure, direct way for a ground path from engine to airframe. What you are grounding (for the most part) is the starter and the alternator, which are attached directly to the crank case. And by grounding the crank case directly to the firewall, you bypass any poor connections at the crank case/mount and mount/firewall points.

Sigh. Yes I'm one of THOSE idiots about ground wire size. lol

Web
 
...by grounding the crank case directly to the firewall, you bypass any poor connections at the crank case/mount and mount/firewall points..

My 53 C180 has braided ground straps from the engine (sump bolts) to the mount.
Can't remember now but I think also has metal grounding straps that wrap around the rubber bushings at the firewall.
The later skywagons have metal wrap-arounds at the Lord mounts.

But as pointed out, it's tough to beat an engine-to-firewall ground wire or strap for simplicity and functionality.
 
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