Hello,
Having upgraded my forks from 80mm to 100mm, I have noticed that the forks are loose in the head tube and when the front brake is applied, the steerer rocks within the tube. Is this because it is an old frame that originally had 80mm forks, or is it me being stupid?
Thanks,
Dan.

Did you fit the forks yourself?

Sounds like play in the headset, might just need tightening up. Loosen the stem bolts, then tighten the bolt in the top of the steerer (not too tight) then re-tighten the stem bolts.

I have tried that and it seems to have little or no effect. Could it be solved if I hammered the SFN in further?

is the stem sitting a few MM above the top of the steerer?

If everying is as tight as it should be it may be worth checking the headset itself. I had similar symptoms after a bit of an off and on investigation I had cracked the headset cup.

The stem is about 1mm above the steerer. I fitted a new headset yesterday too, which didn't solve the problem either.

Did you put the headset back together properly? Definitely not missing any bits?

Oh just seen the edit – just asking cos I once put fitted some forks and somehow missed putting back in the expanding wedge, and was left with lots play in the headset.

1 mm isn't enough. I reckon there's a lip on the stem cap that's just as deep. Therefore, your stem cap bottoms out on the steerer and stops you from applying the proper preload to the headset.

Ok, I'll remove a washer.

Ok, I'll remove a washer.

If you're referring to the spacers and you've only 1mm, you need to add a spacer, not remove.

That's what I meant Nuke 😆

The job is now done, thanks guys, it's fine now. 😀