Hi Garno, sorry I can't help as well the knobs I have are 5mm, It sounds like you need to have a good clean out were you lock it up If you have to use a bar to turn it must be very clogged up.
Sorry what I meant was that as I no longer have the knob to turn it I am using the steel tubing as that is the only thing I have that is small enough to go over the plastic bit and turn it. The block now raises and lowers freely but I cannot keep it at the heights I set it at.
It's my bad as it is a difficult one to explain and it making sense by the end