does gap between top of steerer tube and top of stem have to be 3mm
Just cut a new steerer tube. The gap between it and the top of the stem is about 1-1.5mm. I know that 3mm seems to be preferred, so the stem cap has something to sink on to. When I crank down the stem cap, I get no rattle or weird feeling in the bars. Is 3mm mandatory or can I be lazy and stick with what I have? As I write, I have this weird feeling I should shut up and go back and finish the job I started by taking off another 1.5-2mm. What a PITA.
Seems to me that the only point of the gap is to allow the cap to pre-load the headset before you tighten down the stem bolts. Try it and see if it works with your current gap. If not, file that last mm off.
When you think you are through hold the front brake tight and rock the bike forward and rearward while checking for movement between the steer tube and the lower end of the head tube (shouldn't be any).