Originally Posted by
bitpuddle
Just enough to hold the headset together. As soon as you can apply the front brake and rock the bike back and forth without movement in the headset, you’ve tightened enough. Barely more than finger-tight.
If your headset loosens, you might have issues with the frame prep (head tube not faced/parallel, unevenly reamed, too of steerer not cut at 90 degrees, etc).
+1. I never use torque readings for headsets. Easy way to check for sure is to put everything together, lift bike with the front end pointing down, turn front wheel and let it return to center on it's own. You want it to be just tight enough so the the fork just starts to move freely without any binding. No more, no less. Removing the handlebar so that the cables don't interfere with movement is necessary though. As
bitpuddle mentioned if you're having problems with a CK headset loosening you may need to check everything noted.