Refitted a radiator after decorating but didn’t get any heat to it. Realised it was the lockshield valve closing when I refitted the cap (it screws into the valve).

Cap looks like this:

Is there a special way to tighten this cap without closing the valve?

try screwing the screw in while holding the valve head with some pliers without the plastic cap a few times to loosen off the threads so that the friction of the valve is greater than the friction of the screw threads. Maybe even a drop of oil. Then refit and gently screw the screw in with the cap in place.

Thanks, will give that a go tomorrow.

You squashed the outside when you turned it off didnt you.?

Have a look and you will see the hole is now oval.

Lightly work it back to round.

Done it myself 100’s times, easy mistake.