TRV valve wont fit....

[vivvov]

TRV valve won't fit...Have to replace a TRV valve..thought I'd just relace the head and save on the messing about.. Bought a standard 15mm replacement..went to screw the head replacement on..thread is not standard..inside diameter is approx 1 mm smaller!! See photo attached. Went back to b and q and all the trv heads have same, larger diameter screw on section ...is this normal?
Just want an easy life and not take the radiator off etc...

 

[ShaunCorbs]

Yes there not all the same

 

[vivvov]

Yes there not all the same

Thanks Shaun..retro fitted a couple in my folk's house..had hoped could do the same here...