Ball hitch is 50mm with a neck approx. 25mm then the ball with flat base where it joins the neck (pity can't add photos on this forum). When we collected the caravan the dealer hitched it and the alko arm laid flat as it should be gripping the ball. Now I can't get it down further than half way its so tight I can't pushing it down. I could stand on it but don't want to cause damage if its the discs in the head of the alko hitch.