Chìa khóa vặn van radiator UK 3.7*7mm

Will not free a stuck valve you will need to do that first with some pliers if stuck. Once freed it's a good idea to work it back and forwards half a turn at a time. 

3.7* 7mm Which is standard.

To balance your system, ensure all your TRVs are fully open and you have no solar gain, etc.
It's best to just do it based on the room temperature. I aimed for 18.5 in bedrooms and 21 downstairs.

The print has 70%  infill on the socket with the rest being 20%. Printed in PLA but anything would be fine.