There's a loose ground wire near the coil. I've tired to screw it back onto the fire wall (Yes, I scrapped the firewall and cleaned the area with spray).

But it won't stay. So I must not have the right sized screw. What size is acceptable, and what type of metal? Brass? Zinc? Galvanized?

And should I use a washer between the screw and the ground?

( I mustbe making progress. Neighbors are coming out to look at the car, and I smell of gas and grease even after washing up.... )

 

Check to make sure there is no captive nut on the other side. If so, its probably 1/4" UNF screw that's required. If not, any self-tapping screw would do. Using a serrated washer may help in making a good ground connection. Check for continuity to ground with a multimeter after fitting.

 

I've only seen brass bolts used at the Battery Holdown in S types, not even screws.

I have seen Brass Nuts used in Jaguar manifolds, but nowhere else.

I would replace the screw or bolt with a stainless steel bolt of the correct thread.

 

I don't recall for sure, but I think that uses a #10-32 machine screw. Phillips.
Lin

 

It almost looks like a brass wood screw _ the photos are a bit out of focus and the threads can't be seen clearly.
At any rate the upper part of the shank doesn't look threaded so I would think by the time the under side of the head makes contact with the ground leads, all of the thread will be on the other side of the firewall.
If you try and tighten it, it will just go around in circles.

 

If its on the bulkhead, there should be a captive nut behind to take the screw. Jaguar would have used a cadmium-plated screw, with an anti-shake washer. Nothing wrong with that inside an engine compartment, although brass or stainless is better.