" wrote:

Trying to thread a machine screw into some aluminum.

You could be getting galling on the threads, where the aluminum
gets "gummy" under pressure and stops being a good sliding surface
for the steel.

Yeah, I think so, and apparently others have hinted at that when
talking about lubricant. Thanks.

You might try lubricating the threads before assembly, using
antiseize, or even going to a thread insert (like Helicoil or
Keensert) in the aluminum.
--Glenn Lyford

Much to my pleasant surprise, the primary fastener looks like it
might do the job without any extra strength, so the hole can be wide
enough for just pushing a strong bolt through and securing it on the
other side with a nut. And that will hopefully function as a
momentary/temporary hold in case things start to fall apart.