Todd those are made from 1/8 brass nylon tube inserts.
They were cut down and drilled out to fit the axle. Originally they were made
to work with the front axle holes in the chassis but was to high so
when I added the brass tube to lower it, I made the tube the width of
the chassis and retained the spacers because of the flanged end
and I had already cut them to fit.
They are used front and rear to space the wheels out and were
a pain to drill being thin walled.
I have also used them for axle bushings to use .059 axles in
Aurora chassis.
I got them from the guy who supply's the brass fittings at work but I've
also seen them online. Maybe even at your local plumbing supply
Thanks for looking
Kevin
would have them.