If you are trying to resist shear forces, you want the screw as large as
possible in diameter and you want to avoid having the threaded portion
in contact with the plates in shear.

The bolt strength is not going to be terribly important, the aluminum
has a unknown strength but certainly less than half that of the bolt.

My first choice would be a socket head cap screw in black phosphate
finish but all of those are fully threaded until you get to over an inch
long.

Using a locking nut (Nylock) would be a good idea in this high vibration
application.

You didn't ask but I would be looking at using a fairly high strength
aluminum like 6061-T6 rather some no name, no grade wimpy stuff from the
hardware store.

John Doe wrote:
Note: In order to avoid wasting your time, please be advised that
welding is not currently an option here. Thanks for your
understanding.

I like these skates a lot, planning to make slight variations.

http://www.flickr.com/photos/27532210@N04/2565924423/

The aluminum flat bar is 1/16" thick from McMaster. Seems springier
than Lowe's aluminum and apparently harder so it doesn't clog my
tools.

After sanding the sides of the skate frame and the wheel fork (the two
aluminum flat bar pieces), the pieces are coated and Super glued
together. The wheel ends up precisely aligned.

The skates are taking a beating, that's what they're for. Most of the
forces are shearing type. Twisting pressure is handled by the
flexibility of the frame. I have high confidence in the bond,
otherwise I wouldn't be skating on them (away from cars and with lots
of protective gear). At the same time, I would like to provide a
better backup fastening than the single cheap screw currently being
used.

I would ask about screw placement, but that's probably a fine detail
that I should determine here. The current single screw is where I
think the bond is most likely to fail. I'll probably use more screws.

Again, the aluminum flat bar is 1/16" thick.
The skate frame thickness is somewhere between 1/16 and 1/8". I don't
know exactly, but I think it's a pretty hard alloy of some type.

Which of these McMaster machine screws would be most nearly
acceptable?

... 6-32 or 8-32 Thread

... Black-Oxide Fnsh Pan Head Phil 18-8 SS

... 300 SS Pan Head Phil, specifications met... FF-S-92, QQ-P-35, MIL
51957, and passivated

Is there a matching nut for a rated screw like that? How important is
a nut on the other side of the threaded hole? I'll probably use
superglue to secure the screw.

Thank you.