How to tighten threadless headset? Top cap has no thread The headset on my merida ride 100 has come loose. I watched a few videos to see how to tighten a threadless headset. They all say loosen the stem then tighten the top cap. However on my bike the top cap seems to do nothing, it's not threaded or anything and there's nothing inside.

How do I go about tightening the headset on a setup like this?
On threadless headsets the top cap has a bolt passing through it that threaded into either an expansion plug or star nut that is pressed into the steerer tube.  Tightening the bolt pulls the top cap down which compresses the headset bearings.

I'm guessing your top cap bolt is missing. Any local bike shop will be able to provide a replacement. Perhaps the expansion plug, no longer attached to the top cap, has dropped down into the steerer tube. Have a look down the steerer tube with a bright light. You may be able to fish it out with a length of coat-hanger wire.