Sizing replacement screws

[bristol406]

When removing bits from my car I try to replace any worn or rusty fixing screws when refitting them. With imperial bolts I can usually find the size in the parts catalogue or measure them with calipers and buy replacements but with self tapping screws (drive screws) I seem to have problems finding suitable replacements. The parts catalogue usually only gives the part number and not the size. The one that's foxing me at the moment is the screw holding the ignition switch plastic shroud onto the steering column cover.
Any tips on finding replacement drive screws would be welcome.

[Phil Nottingham]

Rover used some specials but 4 and 2 BA were mostly used for setscrews inside and for electrics although 3/16" UNF were used in some places! I removed the steering column switch on our late P5B so cannot check the screw.

Nos 2, 3 & 4 self tappers were the common sizes

[bristol406]

Thanks Phil. That's helpful.