I am fitting a pocket door and the frame is an aluminium box section.
The instructions assume that the stud wall will be a metal construction
and advises screwing through the metal upright into the aluminium frame
using a self-taping screw.

But my framing is timber so I will need something like a four inch
self-taping screw to fix in the same way. I could get away with
something a bit shorter if I counter-bored the timber first.

The frame is already installed so any holes in the aluminium will have
to be drilled through the timber.

Any suggestions?