advice solicited - metal

"HammerheadFistpunch" (hammerheadfistpunch) 01/02/2020 at 12:25 • Filed to: None

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I have some thin sheet metal in the wiper box behind my firewall that I want to use as 2 of 3 points to mount a bracket and I need help deciding how much kill is enough and how much is overkill.

Here’s the bracket. its mounted on a hard point below, but will also hang on the sheet metal on the back of the bracket. this will hold what you see plus 3 relays and lots of wire.

its 16 gauge steel (the bracket) the sheet metal can’t be more than 22 gauge.

Current plan is to put a backing plate behind the sheet with captive nuts to hold the bracket...but...I need to figure out a way to hold the plate without welding. I was thinking epoxy or oversizing the plate and using self tapping screws just for plate location. But...do you think it would be fine to just use self tapping screws (m6) to hold the bracket to the sheet metal? The idea is that this bracket will be going on and off a few times in its life, at least half a dozen for trimming, painting and wiring.

Thoughts?

adamftw > HammerheadFistpunch 01/02/2020 at 12:38

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You can JB weld or epoxy it, and it’ll probably be fine. Just remember that if you use an epoxy, you’ll never be able to weld it after barbecue it will more than likely release pretty bad fumes. Two nuts/bolts so it stays in the same shape would probably work better, functionally and aesthetically .

You got a tack on there, you can’t burn it fully?

HammerheadFistpunch > adamftw 01/02/2020 at 12:40

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You got a tack on there, you can’t burn it fully?

I don’t want to hook up my electrode to the body at all.  Not comfortable with that yet.

Rusty Vandura - www.tinyurl.com/keepoppo > HammerheadFistpunch 01/02/2020 at 12:41

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I would fit a plate of maybe 0.10" aluminum to the firewall, in the range of 4x6 inches in size , and pop riveted at intervals around the edge, perhaps 0.75" apart. Then carefully pilot drill and use #12 x 3/8" stainless steel pan head sheet metal screws to mount the back face of the bracket to the aluminum plate, the screws purchasing both the aluminum and the steel firewall behind. Probably three of the sheet metal with an inch between them. I would be sure to make the holes in the back of the bracket large enough so that the screws pass through freely.

The aluminum plate can be shaped, or trimmed, to the contour of the firewall as necessary.

DasWauto > HammerheadFistpunch 01/02/2020 at 12:41

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If this is going to be in and out several times I’d avoid the self tapping screw for fear of stripping it. Considering the minimal load, I’d probably just drill the firewall and put in  a pair of rivnuts to make a reusable mounting location. Just make sure to coat the edges of the drilled holes with some paint or primer to avoid rust.

adamftw > HammerheadFistpunch 01/02/2020 at 12:41

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Just disconnect your battery. For short spurts I’ve welded with the battery connected with no issues.

You cant take the whole bracket thing off and weld it on a bench?

HammerheadFistpunch > Rusty Vandura - www.tinyurl.com/keepoppo 01/02/2020 at 12:42

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Thats a little too much work for me, i need this done today

Rusty Vandura - www.tinyurl.com/keepoppo > HammerheadFistpunch 01/02/2020 at 12:43

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Also, looks like that power steering (?) hose might want to rub against the front edge of your platform.

farscythe - makin da cawfee! > HammerheadFistpunch 01/02/2020 at 12:43

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nuts and bolts not an option?

im not sure how much i’d trust self tappers inside the engine bay...what with the vibrations n all

might well do fine...i just tend to trust nuts and bolts more

adamftw > farscythe - makin da cawfee! 01/02/2020 at 12:45

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Self-tappers on the firewall will be fine. Its a Land Cruiser not a top fuel dragster.

adamftw > HammerheadFistpunch 01/02/2020 at 12:47

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Don’ t overthink it, just get it done and crack a beer. Thread cutting bolts will be stronger than self tappers, but you can toss self tappers in for now and change to a the bolts later.

farscythe - makin da cawfee! > adamftw 01/02/2020 at 12:50

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hmmm...thats a good point...maybe im just biased against self tappers

RamblinRover Luxury-Yacht > HammerheadFistpunch 01/02/2020 at 13:06

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The trouble with any self-tapping is that you’ll get at most about a thread of hold, and the immediate grip area can easily pull out distorted. If you were to use rivnuts, the erosion of the thread wouldn’t be an issue and cross-threading would be eliminated, but the tendency of the metal to belly out and stretch would be as bad as ever. However, what you might do is to use rivnuts through the metal *and* a plate on the back. That way, you have a captive nut, it has something to spread the load out, and the captive nut itself is what holds the plate and nut setup in place.

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo 01/02/2020 at 13:13

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Better to have the force coming from behind thin sheet than on the face - and since he’s talking about things to hold the plate on the back , it’s a cinch he can reach the back. What I’d see happening in the “zillions of rivets” scenario is the thin metal pulling out along a crease line through the (weakening) line of rivets. Plus, having layered metal together in the engine compartment can make a trap for moisture. Further still, stainless and aluminum don’t actually play that well together - their corrosion resistances are actually at cross purposes in a way and the two will gall at best.

This is one of those cases where “looks secure as hell!” actually wouldn’t be, so much.

RamblinRover Luxury-Yacht > farscythe - makin da cawfee! 01/02/2020 at 13:16

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GM used to use self-tappers for body work all the time, but they had three things going for them. One, using self-tappers with a cone taper thread that would open the hole up slowly, two, using a fine thread (so there was a whole thread holding), and three , punching the hole with a cross-punch, so there were little tabs at the opening that would end up folding back. Lots more metal getting grabbed than just the edge of the hole.

jminer > HammerheadFistpunch 01/02/2020 at 13:17

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I’d just go with self tapping screws for bracket to sheet metal, it will do it quick and easy. If you have one handy a nut-sert tool will be even better though.

RamblinRover Luxury-Yacht > DasWauto 01/02/2020 at 13:17

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Your idea is good and the basis of mine - I made the possible improvement of mentioning he can rivnut through a reinforcing plate on the back for better stiffness and hold.

HammerheadFistpunch > DasWauto 01/02/2020 at 13:44

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This is how im going to do it.  Good call.

HammerheadFistpunch > adamftw 01/02/2020 at 13:44

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Oh the bracket is fully welded on the backside, not just a tack.  

HammerheadFistpunch > Rusty Vandura - www.tinyurl.com/keepoppo 01/02/2020 at 13:45

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its a fuel breather line and its got a gap and isn’t touching. this bracket isn’t resting on anything at the moment but i am going to do some final trimming, which is why I want it to come in and out.

HammerheadFistpunch > farscythe - makin da cawfee! 01/02/2020 at 13:55

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I want to nut the backside, but I can’t get access to the bolts easily once I button everything up since I need to take off the panel under the wipers to get at the space.

Rusty Vandura - www.tinyurl.com/keepoppo > RamblinRover Luxury-Yacht 01/02/2020 at 14:09

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I have experienced this dissimilar metals thing. Still, that’s how I’d go if it were my vehicle. The rivets are quite a bit fewer than a zillion and are only spreading the load out a bit. The sheet metal screws, quite stout by the standard I recommended, could even be hex-head. I think the car could dissolve over the course of a number of salty winters and those individual bits would still be attached to each other.

Rusty Vandura - www.tinyurl.com/keepoppo > HammerheadFistpunch 01/02/2020 at 14:12

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Okey dokey. You have my $0.02. I have a circuit breaker similar to that mounted on the firewall of my van on a strip of galvanized Tico building clip; I used sheet metal screws, but your platform is larger and heavier. The bulk of the work will be born by the front bracket. But I’m no engineer and NASA rejected my application. They didn’t even send me a thanks-but-no-thanks note.

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo 01/02/2020 at 15:20

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Perhaps not a zillion per se, but still quite a few.

The need to have a foot space for the bracket and some other factors limit the design, but at least the far end mount point would keep it from going up and down much.

I don’t question it working for a good long while, I more question it as the best solution and use of effort.

Mostly, I think I’m “salty” ( if you’ll pardon the pun) because a similar thing to this was done in layers over holes and damage on a Land Rover firewall that I once had to repair, by a previous owner. Rivets, along with screws, welding, brazing, caulk, and epoxy. Each layer compounded the weaknesses and corrosion trapping of the last.

In consequence, I shy away from “rivet a plate to the outside surface!” where steel and firewalls are involved.

The Snowman > HammerheadFistpunch 01/02/2020 at 19:39

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Make a plate with captive nuts and rivet the plate in then it will stay put and be sandwiched on the thin metal when bolted up.

HammerheadFistpunch > The Snowman 01/02/2020 at 19:50

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I just went with rivnuts