In article ,
David_Turner writes
I am trying to find a grub screw for an antique door handle.
It seems to be between 3mm and 4mm in diameter. so i am guessing it is
an imperial size. (1/8inch perhaps)

I could be mistaken, but I'd have expected 1/8" x 40 (1/8 BSW) would be
too fine for that purpose. Could it be 4BA, perhaps?

does anyone know where i could get a grub screw to fit this?

Take the door knob to your nearest engineering suppliers and get them to
try different screws until they find one that fits. Then buy at least
two per handle and a small quantity of matching nuts -- you'll need the
nuts to restore the screw thread when you cut the machine screw(s) to
size...

--
Paul