vxla wrote:
What's the best way to install a window air unit into a 42" window in
a vintage apartment? Should I get some plywood pieces cut to fill the
gap between the unit and the window pane, or is there some other way
that is better?

Thanks.

I'd recommend a 1/2 - 3/4" plywood sheet with a hole cut out for the AC
unit - not boards on either side, but one large board with a hole for the
machine. Bolt the AC unit to the board then screw the board to the window
frame.

Window AC units are relatively heavy and they vibrate. Put in braces and
brace the braces. Any flimsiness will cause failure - a very loud and
exciting failure. While you're asleep. That can't easily be fixed, or even
comprehended, while in your nightshirt.

I commend you for the project. We did something similar and dropped the
electric bill from $200/month (back when $200/month was a lot of money) to
the current $60/month.