Barlo radiators bleed screw

[baumatic]

I have a couple of barlo radiators where the bleed screw head has become rounded at the top. The brass rad keys do not seem to get a good grip on the head and causing the head to get more and more rounded. Is there a tool similar to the rad key which has various sizes of the square notch within the key. I've got the replacement bleeder screws from barlo. Just need the right tool/key.
Many Thanks,
George

 

[ChrisR]

MOst rad keys are concave on the end, before the square is broached out. I've ground a brass one to the shape it would be if put through a pencil sharpener, so it fits the square right to the lowest/best part of the square.
Only other way forward which I know of which can work on these, apart from tiny Vise-Grips, is a Dremel type slittng disc, to make a slot for a screwdriver.