What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Engine mount grounding

iamtheari

Well Known Member
The hardware bag for the engine mount was backordered in my finishing kit so I can’t actually put it on my plane yet. But I did prepare it for installation and came up with a question. The plans say to file four of the bolt holes down to bare metal to electrically bond the engine mount to the fuselage. That will electrically bond the engine mount to the bolt, but not necessarily to the fuselage. I primed the longeron assemblies. Should I sand off primer beneath the engine mount nuts, or does the bolt through the hole in the firewall do a good enough job?
 
There's a grounding strap engine to mount. Section 43, pages 05 and 08.
Thanks. That sets my mind a bit at ease about the results of the step I?m questioning. I am still curious about the step itself, though, which call for filing off powder coat specifically to ensure an electrical bond between the engine mount and fuselage. The ground strap seems like the better solution to that problem and it?s already part of the plan, so why the focus on filing off the powder coat here?
 
The hardware bag for the engine mount was backordered in my finishing kit so I can?t actually put it on my plane yet. But I did prepare it for installation and came up with a question. The plans say to file four of the bolt holes down to bare metal to electrically bond the engine mount to the fuselage. That will electrically bond the engine mount to the bolt, but not necessarily to the fuselage. I primed the longeron assemblies. Should I sand off primer beneath the engine mount nuts, or does the bolt through the hole in the firewall do a good enough job?

I don't see the need to ground the engine mount to the firewall when the Lord mounts will isolate the motor case from the mount.

I would ground from your firewall battery ground bolt, to the motor case with a very good cable. I used welding cable with crimped and soldered eyes. And a second one from another firewall location. If your motor ground fails, the starter ground path will be through your engine control cables.....:eek:

The cable ends did have proper soldering techniques and heat shrink to avoid flex failure.
 
The plans say to file four of the bolt holes down to bare metal to electrically bond the engine mount to the fuselage. That will electrically bond the engine mount to the bolt, but not necessarily to the fuselage.

I think that is a misprint.
The actual reason is because powder coat is not very hard.
The small contact area on edge of the tube will quickly compress the powder coating and reduce the loading on the bolt.
 
I think that is a misprint.
The actual reason is because powder coat is not very hard.
The small contact area on edge of the tube will quickly compress the powder coating and reduce the loading on the bolt.

That makes more sense to me. Thanks. I just looked in section 40A and the tri-gear instructions don't say anything about this. Page 40B-02 step 3 says of the filing, "This will ensure that the engine mount is electrically bonded to the fuselage." Obviously I filed it off no matter the reason, but now I can comfortably build on without sanding primer off the longeron end pieces...once I get the bolts, that is. :)
 
That makes more sense to me. Thanks. I just looked in section 40A and the tri-gear instructions don't say anything about this. Page 40B-02 step 3 says of the filing, "This will ensure that the engine mount is electrically bonded to the fuselage." Obviously I filed it off no matter the reason, but now I can comfortably build on without sanding primer off the longeron end pieces...once I get the bolts, that is. :)

This omission in the tri gear instructions is a serious one. The battery is grounded to the firewall, and the engine is grounded to the engine mount. The engine mount must be electrically connected to the firewall for the engine ground to reach the battery.
 
This omission in the tri gear instructions is a serious one. The battery is grounded to the firewall, and the engine is grounded to the engine mount. The engine mount must be electrically connected to the firewall for the engine ground to reach the battery.

Not the best plan for a number of reasons.
Potential to magnetize the mount. (Fun with whiskey compass swinging)
More failure points, with corrosion prone materials, between engine and ground.
Potential for somewhat higher resistance path than needed when starting (steel instead of copper).

Probably more, but I'm lazy at the moment.

What Dan said, or just pick up some welding cable, as someone else mentioned. Almost as flexible as the braid, and comes standard with a non-abrasive coating. :)

Charlie
 
Remember why you want a good ground on the engine - to run the starter.

The starter has a very nice ground lug on it. The only engine ground you need is from the ground lug on the firewall to the ground lug on the starter. Use the same gauge wire that you run power to the starter.

I?ve seen the Van?s RV-14 engine mount ground setup - and wonder why they would do that when it is so simple to just ground at the starter.

Carl
 
.

I?ve seen the Van?s RV-14 engine mount ground setup - and wonder why they would do that when it is so simple to just ground at the starter.

Carl

I guess there are different opinions on what is simple.
Running two heavy gauge wires all the way fwd to the start is not, in my opinion.
 
I guess there are different opinions on what is simple.
Running two heavy gauge wires all the way fwd to the start is not, in my opinion.

Sorry Scott, I have to disagree:
- The extra foot or so of wire weight is trivial.
- Old school ground straps run from the firewall to a greasy engine bolt are not weight free, but do create the potential for future maintenance issues. We need not perpetuate 1960 Cessna thought.
- Odds are that a plane that has a reluctant starter but a charged battery, the problem is a poor ground return from the starter.
- Considering other stuff a builder can inadvertent do to add ?hidden ground returns? from the engine, failure to have a robust starter ground can lead to starter current finding it?s way back to ground via some expensive piece of avionics.

As I said, just run a #2 wire from the common firewall ground to the nice ground lug on the starter and build on.

#2 welding cable is amazingly robust to abrasion and heat, and inexpensive. I use #4 to connect each battery and #6 for the alternator output.

Carl
 
Like I said.... differing opinions

Sorry Scott, I have to disagree:
- The extra foot or so of wire weight is trivial.

Get out your tape measure. The typical cable path from the battery/firewall area to the starter motor requires somewhere between 3-4 feet of cable (and the weight per foot of 2 gauge copper wire is not trivial), plus the biggest cabling pain in the engine compartment is properly anchoring wires / cables to the engine around the top of the engine sump. Running a ground cable all the way to the front doubles that.

- Old school ground straps run from the firewall to a greasy engine bolt are not weight free, but do create the potential for future maintenance issues. We need not perpetuate 1960 Cessna thought.

I don't think the starter area of a typical Lycoming is any cleaner than the aft rear of the engine case.

- Odds are that a plane that has a reluctant starter but a charged battery, the problem is a poor ground return from the starter.

I agree, but that problem can exist with any high power cable connection.... regardless of whether it is attached at the engine case or directly on the starter motor. With four bolts mounting the starter, I have never seen a ground problem at that location.
 
Back
Top