This 1/2 inch allen key / hex spanner is used to install (or remove) the tails of union type radiator valves into a radiator.
Made from high tensile steel, length approx 135mm.
![Example of the radiator valve tail key in use Example of the radiator valve tail key in use](https://www.stevensonplumbing.co.uk/images/source/Tools/keys/tail_key_example.jpg)
This IS NOT the tool you need to operate the air vent at the top of your radiator (we keep bleed keys for that in our store).