Bicycle Mechanics - Head set still loose

I replaced my 1 1/8" fork on my MTB. Everything went well except the head set is still loose. I inserted the star nut the exact same distance as my last fork (after cutting the steering tube), replaced all the spacers, tightened the head set screw, then tightened the stem on (really tight/bad, I know), but still there is movement when I hold the front break and move my bike back and forth. What could be wrong?

Is is important to know I switched from sprung to unsprung?

do you have enough spacers in the stack? missing pieces in the headset?

do you have enough spacers in the stack? missing pieces in the headset?

all of the head set is there, nothing is missing. and all my spacers are good. any ideas?

all of the head set is there, nothing is missing. and all my spacers are good. any ideas?

The new fork has marginally more steerer than your last one - you don't have enough spacers. As well you may want to triple check that the results of your headset check, really is indicating a loose headset, and not something else. Pay attention when you do the test.

Pull off the top cap. The steerer has to end 2-3mm short of the top of the stem or spacer, whichever's on top. If the steerer is flush with the stop of the stack, the top cap will bottom against the steerer instead of pushing the stem down to compress the headset.

A few ideas:

Check real closely and make sure all the spacers are seated together properly. Sometimes spacers from different manufacturers don't seat together properly.

Loose ball bearing system and you lost a ball?

Take the top cap off and compare the steerer of the new and old forks to make sure they are the same size. Sounds simple, but make sure you didn't get a 1" fork mixed with a 1 1/4" or a 1 1/8" fork. (you'd be surprised at the stupidly simple things I've goofed up!).

Reseat the whole thing. Reloosen it, then lift the front end off the ground while supporting it under the fork crown (put hand between the wheel and where the fork blades meet). Push all spacers into place and check that all are seated together properly, then push the barstem down on top of the stack (maybe get a friend to do this with a finger or two to hold a little pressure on it), then retighten.

I bet the star fangled nut is not locking.

Probably NOT the problem, but surfrider's mention of a lost ball reminded me of where a lost ball can end up, between the steerer and head tube wall, making headset adjustment pretty impossible.

As long as the top cap bolt isn't running out of threads during tightening (most are fully threaded), I agree with the others here that most likely the spacers are insufficient and thus the cap is bottoming on the end of the steerer before bearing pre-load is achieved during tightening.

If the headset was ridden for a while off road with looseness, a bearing cup may have gotten hogged out enough to allow radial "floating" of the bearing in the cup. My FSA lower cup suffered this exact malady, which could not be cured through subsequent adjustment.

Sweet! so, just to make sure, I should trim down the steerer 2-3 mm

FBinNY has it right. you need a spacer above your stem and that spacer must be taller than your steertube so your top cap is pressing on the spacer instead of the steertube. tighten your topcap first then your stem.
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Make sure your front wheel-bearings aren't loose. That would show up when rocking the bike forwards & back with the brake on as well.

Make sure your front wheel-bearings aren't loose. That would show up when rocking the bike forwards & back with the brake on as well.

That reminds me, any suspension fork has a bit of play as well!

Just got in and cut the steering tube down 2 mm, replaced the stem, reset the head set, and... It worked beautifully!