1. Where can I get a new one for a '64 Valiant with 9X2 drums all around?
2. What are the different sizes? I'll be switching to front disks from a '74 Duster some time in the future. Are they the same?
3. What would cause it to come off? I haven't been racing, autorcrossing, or playing "Rat Patrol", haven't had the wheel off, and haven't really done anything lately with it lately but drive to and from work.
The story: My driver's side cap is missing ... again.
Last week, I happened to look at the original when I was checking air pressure. (Don't have a full set of wheel covers on at the moment) Nothing unusual, I just remember seeing it.
After a store run, I walked out to the car, and the cap was gone.
I grabbed one at the nearest junkyard - the last one from the last '60's Mopar in the yard, a '66 Valiant. I cleaned it up and put it on.
Yesterday, after work, THAT one was missing.
There are a couple of caps in the "HELP!" section at the parts stores, but they do not fit.
Ideas?

1. You don't. They aren't being made.
2. No, they are not the same size. The readly available ones are for the 10" drums.
3. After 40 some years, they start to loose there grip and just fall off.

At least Year One still has them in their online catalog. See:

http://www.yearone.com/serverfiles/part ... A18AC56282

I also have some pretty beat up ones from a 1961 Valiant with
9" drums if you want used ones.

I need something soon. Used would work for now - I'd pay some attention to the fit this time.
How much to send me a couple?

Everybody "has them" in their catalog. But when you actually try to use them, they do not fit. The end of the spindle strikes against the dust cap. It will not seat correctly. The spindle is about 1/8" to long. I have tried every conceivable outlet for these caps, including trailer supply outlets, nobody makes the correct dust cap for the early 9" drums.

The used ones I have from the 61 - After cleaning the grease out of them
show wear - on the inside where the spindle has rubbed. These caps have
obvious signs of being "tapped" on.

The clearance between the spindle end and cap on the original caps
must be very tiny.

I can see how they could work loose also - since the pressure from
the spindle on the unevenly bent surface would just rock them out
as the wheel spins.

Will have to refurbish these before use - tap them back out from the
inside !

Will somebody please carefully measure the O.D. of the part of the cap that fits into the hub and tell me if it's 1-9/16" or some other dimension?

1.55 +.013 inches in diameter
1.55 +.027 inches in diameter 90 degrees from 1st measurement

avg 1.55 + .013 + .007 = 1.570

and 1 9/16 is 1.5625 inches

Just a Hair bigger, but Pretty Darn Close to 1 9/16