On Oct 8, 3:29*pm, John Doe wrote:
Something thin to put between two pieces of 1/8" thick 1 1/2" wide
aluminum flat bar that are bolted together. A bearing that will
allow the two pieces of aluminum flat bar to be held firmly together
but move (the only way possible) against the spacer between them
with as little friction as possible.
[slipperiness and weatherproofing are important]

Bearings work best with hard-against-soft (this provides for
embedding of grit into the soft side), so plastic washers
are a suitable solution. Delrin and some polyacetal plastics
are intended for long life as bearing material, should be available
in washer form. Brass (a washer, or even just a disc cut from
shim material) will also work, might be better if the aluminum
surfaces are hard (anodized). Even stainless steel might
be OK, if the aluminum is not anodized (the aluminum is the
soft side, then).

The 'bolt' will rub the sides of the hole, and some loosening of
a nut can occur, so various tricks are common in this kind of
pivot. Easiest, is to fill the hole with a shoulder bolt, and
glue the nut in place. Shoulder bolts are expensive, though.
Second easiest is to fill the hole with a (slightly short) tube
and use nut/bolt through the tube to keep the joint together.
Washers (and maybe Belleville springs) are important,
use them freely. One side of the bolt could take a lockwasher,
the other then has to have slippage (like, use a brass washer/
steel washer/lockwasher/nut).

I'm a big fan of riveting, but good rivets are hard to find, like
cheap shoulder bolts. It's probably easy to order thousands,
but try to find one the right size when you need it...