MrChris79 Quite a big one, perhaps 38mm, can’t remember. If the old heating element is gone, undoing it will effectively drain the boiler.
Touch of vaseline…just find a mate with piles, don’t do the element too tight, the o ring needs to seal, but mustn’t be crushed. I pretty much did mine a hand tight by holding the socket, not using a tommy bar/ratchet handle!
Switch the machine on and it will refill….
You might need to remove the old element with an impact wrench if it’s been put on super tight…and definitely remove the boiler to do the job if there is any problem undoing the heating element, you don’t want to twist anything.