Don Foreman wrote:

That looks like a compression fitting, not a flare, particularly since
you mention a small brass fitting inside. If it looks like a little
brass ring with tapered front and back, it's a compression fitting.

That is indeed what it looks like. On the pilot light side of the tube,
this brass ring is loose around the aluminum tube. On the other side it
is rigid.

I have added a couple of photos of both ends of the tube on my site.
The top photo is of the bad one, the bottom one is the good one. You
can clearly see that the brass ring in the top photo is shorter
(compressed more?) than the bottom one. The ring is loose around the
aluminum tube. In the "good" end, the brass ring is tight around the tube.
http://www.abrasha.com/misc/heater_536.htm

Not a good choice for gas, particularly with aluminum tubing, but if
that's what you have, then you're stuck with it.

Get a new brass ring at the hardware store. Cut about 1/2" off the
end of the tube with a tubing cutter, install the threaded part and
then the brass ring, reassemble and torque it just until it quits
leaking.

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