Abrasha wrote:

Wouldn't I need to somehow make a tight fit between the new brass ring
and the aluminum tube first, before I reassemble the parts?

Abrasha
http://www.abrasha.com

No, that's part of the "compression" in "compression fitting" -- as you
tighten things the double-taper on the ferrule makes it shrink (some are
split, IIRC) so that it squeezes onto the tube and makes a seal.

I don't have any experience with mixed-metal compression fittings but
I'm deeply suspicious of such a critter in an application where the
temperature swings back and forth -- how long did the current
installation last, and could the aluminum tubing be an incorrect
replacement for copper?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com