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"Faux stud mount" for avionics boxes and such?

N546RV

Well Known Member
As I continue to nail down placement of remote avionics boxes behind my panel, I'm wondering about mounting options. I will have four devices mounted to the aft side of my -8's baggage bulkhead; this in turn means that I'll have some fasteners projecting into the forward baggage area.

The most "natural" thing to do here would be to simply install nutplates on the forward side of the bulkhead (in the baggage area), and install screws for the boxes from the aft side. The idea of these nutplates sticking out in the baggage area doesn't bother me a ton, but it would be nice if I could avoid having little snaggy bits sticking out in there.

So I was wondering whether it'd be acceptable to instead do what I feel I can best describe as a "faux stud mount." The idea would be to install machine screws from the forward side, and secure the screws to the bulkhead using, say, AN316 thin nuts. The avionics component would then slide in place over the protruding screw shafts, and be secured with a nylock nut.

A crappy five-minute diagram of this setup is attached below. (note: I didn't take the time to depict washers in the stackup)

Is this idea 1) acceptable or 2) oh no no no don't do that?
 

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Looks fine to me. Some might just use a plain spacer/washer if you need the standoff, but with multiple fasteners all wobbling around at the same time, the thin nut might save some cussing. It could complicate torquing the final nut though. Are your torquing against the screw or the thin nut kind of quandary.

If you just need to retain the screw, you could simplify and just use a thin plastic or paper "keeper" like they use on electrical outlet screws.
https://www.amazon.com/Nylon-Retaining-Washer-White-Inch/dp/B000OAD4AO?th=1 (not the right size)
 
Only real issue is that there is a decent chance that the screws will spin, requiring a second person to put a screwdriver on the head when tightening or loosening the nuts. They make studs that are welded to 1" steel discs, but can't remember what they are called or whether they make them as small as 3/16". You could drill holes in the disc to attach with rivets.

Larry
 
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Why not mount the nutplate to the equipment side of the panel, and use flush rivets in the baggage compartment? With a carefully chosen screw length, you wouldn't even need a screw hole through the panel.
 
You could also rivet some Z-channel to the aft side of the baggage bulkhead (flush rivet heads on the forward side) and then use whatever nutplate arrangement you like on the resulting flanges on the aft side.
 
The boxes you mount will stay there for many years. Using a screw in a dimpled hole forward with a washer to offset up around the dimple aft will look good and be very secure with a lock nut aft. Domed screws would not need a dimple keeping it simpler. Yes you might need a helper but it won’t be often. If you don’t want that then the domed sealed nutplates used in tanks are an expensive smooth screw cover alternative.
 
Hi-Lok Fasteners

You could achieve this using Hi-Lok fasteners. You would have to drill and ream the holes to the proper interference fit. Then you would need to install it. At work we have special pullers to pull them through. But I know that they can be installed using a standard rivet gun if you’re careful. Knowing that some day you may want the avionics boxes off, use an MS21042 nut, and be sure to back up the torque with an Allen wrench.

These are not inexpensive fasteners, but they do exactly what you are asking for.

https://www.skygeek.com/hi-shear-brand.html
 
I use that solution occasionally and it works well. Although, I use a standard flat nut and a lock washer instead of a thin fiber nut since it has a flatter surface.
Also works well mounting stuff inside the fuselage at the wing root, through the fuselage side.
If you are working alone, it is a real help during maintenance.
 
Avionic tray attachment

I used a couple of small piano hinge sections, so the tray can be easily removed.
 
I used a couple of small piano hinge sections, so the tray can be easily removed.

I would not consider mounting $10,000 avionics via a piano hinge. They have a fair amount of slop and would allow an amplication of vibration, along with added shock loading as it hit the end of travel on each vibration. I would prefer a rigid installation.

Larry
 
I think I would use Matt's idea and just rivet a bracket of some sort to those panels. Then you could add the mounting features to those brackets.
 
I've used the OP's method extensively, albeit with flat nuts, internal tooth lock washers and flat washers.

The key to success is in ensuring the fiber lock nut subsequently used to affix the avionics to the "faux stud" has been run through with a screw a couple of times prior to installation.

They will never come loose in service but will have slightly less holding torque than the flat nut and internal-tooth lock washer on the "faux stud".

It's rare that the stud will spin and common that the fiber locknut comes off without any problem and no need for a helper on the other side of the bulkhead.
 
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