I need to bleed a radiator.

I have the standard bleed key, with a female 5.8mm square (I have measured it).

The problem radiator has a square bleed screw that is slightly smaller than 5.8mm and the bleed key does not grab it.

I believe the bleed screw may have been worn out and that is the reason why the key does not work. However, the edges look in reasonable condition.

I have tried with bent nose plier but it is not enough grip.

Any ideas how I can turn the bleed screw?

Thanks,

Antonio