5/16" according to this too

Would a 1/4" driver, as used in battery drills be any use ? Loads of styles and cheap.

If you need this sort of info in future, you might help us to respond by saying what style of fastener you are considering. A cap screw will take a larger size than a button head or a countersunk head, and a grub screw (US = set screw) will be smaller still.

One details about hex socket drive fasteners is that there are no special keys needed for Whitworth, BSF, or BA. They all rely on the same set of key sizes as those for UNF, UNC etc.

Cheers, Tim

Edited By Tim Stevens on 21/09/2016 18:29:07

for the record Bandersnatch's post appears to be a facsimile of the cap screw page from ASTM B 18-3 and should be so recognised.

Dunno – I copied it from Gould Fasteners "Comprehensive Fastener Guide". No reference to the ASTM spec.

But if it keeps the peace ……

In my Pitmans book on aircraft spec bolts AN standard 5/16" UNF uses a 7/32" key, there is no size in that series of bolts, internal wrenching that uses a 1/4" key. I see that the Unbrako bolts use a 1/4" key for 5/16" UNF/UNC bolts.

Ian S C