On Tue, 7 Jul 2020 06:05:54 -0700 (PDT), wrote:

I have 2 gang box and I need to expand it to 3 gang.
Looked at several videos on U-tube and others, but they all suggest removing the existing box and replacing it with 3 gang plastic.
Don't like to use plastic because the threads strip to easily.
I thought the industry made it simple to add another gang to the existing older gang-able romex style steel boxes ?

The existing boxes are probably something close to this:
https://www.homedepot.com/p/Steel-Ci...-25R/202597292

The building is 40 years old. The boxes are gang-able, but how do gang and extra w/o removing the old ?

Thanks

You are probably going to end up with a drywall patch, just to get to
the screw on top of the box (or bottom, depending on how it was
mounted).
One of those ratchet right angle screwdrivers with the removable 1/4"
hex shank bit might be your best bet. With luck it might not be that
big a patch.
Don't take that screw all the way out, just far enough to get the end
plate off. Enlarge the hole and put in the extension box with end
plate attached. It is a wiggle and pray trick to get the ear aligned
and run the screw back down. Do yourself a favor and tie a string on
the extension box so when you drop it inside the wall, you can get it
back. Loop the string through the holes in back, cut it off when you
are done.

At the end of the day you may be asking why you didn't just cut the
ears off the old box, pull it out and install a new "old work" box
unless you have a lot of luck and great dexterity.