Another cracked firewall

[SteinOnkel]

Hello,

yep, it's pretty bad on my '78, standard run of the mill clutch cable donut crack.

Question:

What is the minimum stuff I need to remove to get in there and weld up a new plate? I'm thinking booster, mc, intake manifold, pedal box?

Anything else?

Thanks
Steiny

[wetminkey]

I think 81spidermatt has dealt with this recently. Perhaps he will post, or you may have to PM him to ask,...
I usually wish I had the manual tranny, instead of a TH180, except when I hear about firewall cracking,...I understand it's not too unusual for Spiders. Is that true?
Best of luck with your repair!

[Odoyle]

Your thinking right, booster, mc, pedal box, prob intake manifold will have to come out, I did this repair on my 77 in 2017 so I'll share some photos.
Use a drill to remove the spot welds on the doughnut shaped metal plate.

I used piece of galvnized steel from the Home Depot, then stripped the coating with muriatic acid and drilled a hole in the center.

Used an eastwood MIG welder, I first welded the doughnut shaped plate to the new rectangular plate, then welded the whole thing in. After a grinder and some seam sealer, it looks invisible and is stronger than ever. Also welded the cracks from the driver footwell side and grinded them down before putting the pedal box in.


My mechanic said this can happened when the grounding cable below the steering box becomes lose and the clutch cable then becomes the ground between the chassis and engine and begins to melt the cable. I examined my 83 spider and noticed that the ground cable is bolted to the lower starter motor bolt, which seems like a better spot, if anyone is looking to relocate from the trans bellhousing.

[SteinOnkel]

Thanks for the pictures.

Looks like a solid day of wrenching. Nothing we can't handle.

Cheers
Steiny

[JohnMc]

I made measurements and template and cut out a large piece of sheet metal that was 15g. I found the Harbor Freight shears actually cut 15g. Also figured the size of some holes and bought some hole saws and cut by by drill press. Made it large enough so that the it was doubly held in place by the four bolts of the power assist brake. Then MIG welded it in place.
I did remove the power assistant brake, the master brake cylinder, and the engine head. Really needed to get in there as I am not a great welder and needed room to do it right. Had it large enough so that it overlapped and could not slip through anywhere. Then painted and cannot even see it now.
Works excellent.
I also found that in installing the new clutch cable to the clutch pedal was a pain but worked when I took off one of the rubber drains that is in the windshield wiper motor area to the back of the engine bay and could look through and really see what was happening. Those rubber drains are very expensive so handle with care.
also installed an additional ground cable from the engine to the frame (put it on a bolt to the oil filter assembly) - but my failure was apparently just fatigue on top of the previous owner trying to fix with a rivet through an aluminium plate and adhesive.

[SteinOnkel]

We may have come up with a better and easier solution. My firewall was so roached, that tacking together the flimsy sheetmetal would not have done the job. Instead we cut out all of the broken bits to make some room and used 1/4" steel plate and bracing to make an attachment to the brake booster bracket. Works absolutely killer and this will last the next 50 years.

The beauty of this approach is that all welding is done on the bench.

Pictures here: https://imgur.com/a/clFGvOA

Cheers
Steiny

P.S: As for the ground cable being the culprit, I don't buy that. The clutch cable has 1/4" rubber insulation around it and at the point where it passes through the firewall it's got a rubber grommet. Now way no how will that let electrical current pass through it. Besides, the ends of the cable would rust first, not the firewall.

[18Fiatsandcounting]

P.S: As for the ground cable being the culprit, I don't buy that. The clutch cable has 1/4" rubber insulation around it and at the point where it passes through the firewall it's got a rubber grommet. Now way no how will that let electrical current pass through it. Besides, the ends of the cable would rust first, not the firewall.

I think an alternate ground path for the starter through the clutch cable is thus: Battery is connected to chassis. The metal ball on the end of the clutch cable rests against the metal clutch pedal lever. The other end of the metal clutch cable is connected to the metal fork that goes into the bellhousing, which then rests against the throwout bearing which rides on a metal collar attached to the bellhousing. The bellhousing is connected to the engine block (starter). So, in the absence of a good ground from battery or chassis to the engine block, current flows from the battery to the clutch pedal, through the clutch cable to the clutch fork, and then ultimately to the starter. It's not a very good current path, but it might "work". Sort of.

-Bryan

[SteinOnkel]

Bryan, that seems plausible. Although I'm missing a ground strap on the transmission (will be adding that later today) I saw no evidence of any sort of corrosion on the old clutch cable. The firewall donut a bit, yeah, but not to the extent of electricity being involved.

[18Fiatsandcounting]

Since my battery grounds always seemed to be somewhat weak, I ended up with 3 grounds for my battery/engine/transmission: 1) Battery negative cable to the bolt connection point on the frame, 2) With a large wire, from there to a lug on the passenger side engine mount (the bracket attached to the engine), and 3) The one you noted, the wire from the bellhousing to the chassis.

Which reminds me: #3 in the list above is just the end of the wire crimped into a large "wire lug end" that is welded to the underside of the frame. Given that this connection below the car sees water, dirt, salt, etc., it would seem susceptible to corrosion. One of these days I'll go in there and see if I can come up with something better, or at least coat the existing connection with undercoating or the like.

-Bryan

[FordPrefect]

Struggling with some clutch problems recently, and when things seemed to be looking up the firewall have way on my baby, too.

I like the idea of welding on to the booster bracket. Any advice, tips,etc? How precise does the location and angle need to be?

[SteinOnkel]

It's a pretty straightforward job.

You don't have to remove the master cylinder, loosening it is fine. It's a bit of a squueze to get the booster out, but nothing crazy.

You then basically make the hole in the firewall bigger. Just cut the whole donut thingy out. Then you make your bracket on the booster out of 1/4" steel, weld it on and use a step drill to cut a hole into it the right size.

That's pretty much it. The angle and exact position is not so critical. There's a fair bit of adjustment room in the cable anyways.

Been working great so far.

Cheers
Steiny

[FordPrefect]

Thanks!