Dear All
I have just changed the stem on my MTB. I have tightened everything, I think, but there is now a good bit of play back and forward just above the bike frame headset tube. i.e if I put the front brake on the bars/ steerer will move forward/ backward a couple of mm.
Regards
Sh*t Bike Mechanic

Undo the stem bolts and nip up the topcap bolt til the plays gone and tighten the stem bolts back up,
Don’t over tighten the topcap bolt or you’ll crush the headset bearings, it only preloads the headset..
If your new stem is a shorter stack height than the old one you might need another spacer to preload the headset properly as the stem and spacers need to be a few mm higher than the fork steerer to take up the slack..

Not easy to explain (try You tube) but you’ve tightened things in the wrong order. Set the bearing pre-load first, then the stem clamp screws.

Or, you did it in the correct order but the new stem has a shorter clamp, I.e takes up less steerer. In that case, the top cap can bottom out on the steerer before it’s loaded the headset bearings, you may need an extra 5mm (or even 10mm) spacer, or break out the hacksaw and cut a few mm from the fork steerer.

I see…. I had tightened the stem first and not the top cap. Idiot. Thanks for the help. It is good to know there are a team of crack bicycle doctors on call late Friday night instead of everybody being out enjoying themselves in the pub.