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post #1 of 4 (permalink) Old 04-13-2017, 12:00 PM Thread Starter
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Is this a firewall crack?

I was inspecting my tbird a few days ago and one thought had occured to me after our last race -- if I'm putting enough stress on the car with the new stiffer dampeners to potentially stress the worn out wheel studs to the point of breaking(which will all be replaced once I decide if we are doing a hub swap or not), I wondered what else has worn out on the car.

Anyway, I looked over the car and found this one crack that I suspect might be a stress crack.
Q: Can someone confirm this for me? On the front side of the firewall, this crack is covered by the metal that forms the inner fender so I don't see a crack on the other side.
Q: How should I fix it? Ignore it? just lay a bead with my mig welder? I think I have .035" wire loaded in my gas MIG setup. Is this good enough or should I step down to .025" wire (have a small spool and the proper knurl but just need to swap them out.)

I also looked under the driver area and there dont' seem to be any cracks visible from the inside (no dash so this is easy) or from the engine bay. No flex felt with the pedals (other than the flex b/c of the relatively crappy throttle spring).

PS. Wheel Bearings might be a distinct possibility which will also be replaced once we decide about the hub swap.
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post #2 of 4 (permalink) Old 04-13-2017, 12:33 PM
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Yes that is a firewall crack. Just welding it won't work because it makes the metal brittle, and it will crack again. To prevent it from going any further, drill 1/4" holes at the ends of any points (it looks like you have 3 on that crack, 2 on the left side and one on the right), then weld across the crack, and finally plug weld the holes you drilled.

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post #3 of 4 (permalink) Old 04-13-2017, 12:39 PM Thread Starter
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Quote:
Originally Posted by MadMikeyL View Post
Yes that is a firewall crack. Just welding it won't work because it makes the metal brittle, and it will crack again. To prevent it from going any further, drill 1/4" holes at the ends of any points (it looks like you have 3 on that crack, 2 on the left side and one on the right), then weld across the crack, and finally plug weld the holes you drilled.
Just so I understand correctly, plug welding requires a sheet of metal placed on top.
Since it can't go directly over the crack b/c of the welding you suggest, do I need to make a boomerang shaped piece of metal to go over the three holes and then plug weld from inside the fenderwell?

Plug Welds and MIG Spot Welding

Also, how thick should the boomerang shaped brace be? sheet metal thickness, enough to weld plugs without burning through, or something thicker? I have some extra 3/6" plate from the subframe braces but I think that might be a little too heavy.

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Last edited by S4gunn; 04-13-2017 at 12:44 PM.
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post #4 of 4 (permalink) Old 04-13-2017, 02:01 PM
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Maybe plug weld was the wrong term, since you aren't attaching 2 pieces of metal. I just meant weld the drill holes up last, and build it up from the perimeter of the hole in toward the center until it is filled.

-91 Cougar LS, coming soon, complete overhaul with a 427" Windsor.
-90SC 5-speed, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, soon to be transplanted into...
-90 XR7 5-speed black on black w/sunroof
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-70 Torino GT Fastback project car. Needs EFI and a manual trans, but I'm not sure what motor to go with yet.
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