BEST question

IF its a screw, then something under 1/8" width STRAIGHT What you want is an electricians type, but most new hardware guys have no clue what that is...

a norman straight screwdriver, flares out in width from the front flat.. if the blade is 1/4" wide, then it gets wider just behind, to perhaps almost 3/'8".. whereas an electricians type stays that width all the way.. you are looking for like a 3/32" round..with a straight tip. and maybe its an allan? --Shiva--

if the blade is 1/4" wide, then it gets wider just

why do they get wider like that??

who knows, strength for 'normal usage'? maybe??? I am guessing when first made, you take flat bar stock and a forge, hammer it down, then grind it down to the width/thickness for the screw slot.. but electrical stuff hasnt got the room, so they made them straight shaft to get in tighter holes.. also easier to insulate the body, so as to not cause a short.. Its hard to insulate a differing diameter/shape successfully, but pretty easy to 'sleeve' a round bar consistent diameter --Shiva--

That sorta makes sense, when you slam it, it expands, then you trim away.

I recall wrapping tape on screwdrivers for that very reason.

As good a theory as any!