There are two grub screws (blue arrows) that need a small Allen key. If you undo the large bolts for the silver cylinder that 'clicks' to lock the stem in place (red arrow shows one side, other bolt is hidden), you can adjust the grub screws to move the cylinder up or down, to make the joint looser or tighter when it closes (see red arrow on the second image), then tighten the large bolts again once you're happy - you need to get the right balance between having a tight join when locked, and the cylinder being easy to click into place.



Stem locking mechanism showing grub screws (blue arrows), and the bolt for one side of the cylinder (red arrow).

Red arrow pointing at the grub screw pushing against the cylinder to tighten the join once locked.