While restoring a WWII jeep, I learned the importance of proper grounding to run all the electrical accessorties and to get a good connection for the starting system.
Proper cables, star washers to bite into the metal, etc. Every connection had to be ground down to bare metal, and in some cases, tin applied to provide a good ground. Proably 20 or so grounding or bond straps to complete the connections.
On my 1915 frame, I see one connection for the negative battery cable and that is it. Presume the engine motor mount arms to the frame get the engine grounded.
As I'm cleaning, priming and painting my frame, what areas need to be bare metal, if any, to make good negative ground connections.
I'll hang up and listen. Thanks in advance.
What ever areas you want to use a ground. Your 15 and my 16 didn't have an electrical system so you choice is good as any.
I'll be adding turn signals and tail lights to my 16 so I'll pick a couple of spots, remove paint drill and attach associated ground wire,then repaint. I like using period hardware because it will appear correct to most people except for those cranky old purists who will see it and gasp with shame.
The inside of the bearing where the pan snout is clamped to the front cross member is the engine ground. You need to make sure the engine pan is not painted there. You should not paint the inside of the bearing. You should assemble them with grease, as per the shop manual instructions, so that the engine snout can allow the frame to twist without bending the engine pan or breaking off the rear pan arms.
That is all you need to do. The bolts at the pan arms will cut through the paint and make a ground there without any help from you. You will have additional ground paths at the engine pan bolts, the throttle linkage, the spark linkage, the drive shaft, clutch link, etc, etc, etc.
It appears you have, or will have, a starter. It is the big current-user, and proper electrical path to it is essential. If you won't have a starter, ignore the rest of this message.
The first thing is to be sure the starter cables, from battery to switch, and from switch to starter, are the proper size. The modern cables made for 12 volt cars will not do! You need at least #0 (pronounced "One-Ought"), and #00 (pronounced "Two-Ought") is better,
Then, the starter switch (the one you mash with your heel) is always a weak point. You should consider replacing it with a starter solenoid (a 6-volt solenoid is available from parts houses, listed as for 1930 or so Fords, or from a Tractor Supply house even cheaper), and you let the foot switch energize the solenoid.
Then there's the negative path (Model T's are negative-ground).
The ground cable or strap from battery to frame must also be big and beefy. Old woven straps would be OK, but you need to be sure the copper hasn't corroded inside the end fittings. As you know, the connection to the frame must be clean, bright, and tight.
The connection from frame to engine can be a little iffy, if everything has been well painted. I prefer to install a cable (a modern 12 volt cable will do here, because it's only a supplememt, and will not need to carry the whole load) from the engine to the frame. I use one of the bolts on the 4th. main, and one of the bolts holding the bearing at the end of the brake lever arm, but any good places will do.
One more thing -- If the engine or starter was painted, the connection there is ruined. Before installing the starter in the engine, its front plate and the matching surface on the engine, should be scraped and sanded bright and clean.
If you do all this carefully, you'll never be sorry, and you will never be tempted to go to an 8- or 12- volt system, because everything will work better than you expect.