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I've fitted stems before where the top of the steerer tube is 3mm below the top of the stem; pre-loading pulls the top of the steerer up above the top pinch bolt and everything's been fine.
Currently I have a tiny amount of steerer & a spacer above the stem but want to tinker and raise the stem slightly & go back to the 3mm steerer below the top of the stem setup.
The PDF instructions I have for my Thomson x4 stem actually state that a gap of no more than 3mm below the stem is required but I've noticed that the instructions have been revised and that the current download on the Thomson website now states that 3mm of steerer is required above the stem.
I'm assuming that going back to the old style arrangement is ok and if the pinch bolts are correctly torqued I'm not going to wrench the stem off but I thought that I'd check (the height gain will only be a few mm so it's not a big deal).
It's a steel steerer (Singular Swift) so there's no worry about uneven stress on a carbon steerer.
As a relative amateur in these matters, I'm going to open myself up to potential ridicule..
Surely the top of the steerer must sit slightly below the top of the stem/top spacer in order for the top cap to clamp everything down nice and securely?
A steerer 3mm longer than the stem top sounds like a recipe for rattles after the first ride.
I think perhaps stem manufacturers have become risk averse and would rather you used an extra spacer on the top rather than have a (minuscule) risk of the stem working loose - knees be damned.
I would personally just do it the old/usual way.
Edit: having actually looked at the X4 stem, the top clamp is very narrow and high. I can see how the top clamp could be compromised with a only a slightly bigger than specified gap (say, 5-6mm not 3mm, with the steerer lower than stem).
Sounds ok. But revised instructions for the same product suggest some failures. Can you not find the spacer height some other way? Try rotating the bars a touch?
The top of the steerer tube must be below the top of the stem OR the spacer on top of the stem. The normal gap is around 3mm, you can probably have 5mm but not much more and you don't want less than 3mm or you might not be able to get the headset bearings loaded properly.
Or buy a DMR Defy stem and give yourself even more room to play with.
Might look a bit gnar on a singular swift though
A steerer 3mm longer than the stem top sounds like a recipe for rattles after the first ride.
Carbon steerers explicitly state a spacer should be placed above the stem. For alloy steerers, I would not be worried with stem a few mm above the steerer provided both stem bolts are clamping metal! 3T stems only have one large bolt. That helps if the steerer has been cut a little short.
I've run my X4 stem for years on my Enduro bike and never run a spacer on top. *alloy steerer
I'd be happy to run it the traditional way, although I do like a spacer above so I have options for tweaking the position. That said I'd sell the Thomson and buy pretty much anything else.
I've seen some very specific instructions for certain stems and carbon steerers, not the 2-5mm it used to be.
But to say that you have to have a spacer above the stem is just ugly and pointless for what's meant to be a high end component.
Well the deed's done and it only gains a couple of MM but we'll see how it goes; it might be new stem time.
With pre-load applied there's about 1mm between the top of the steerer & the top of the stem so hopefully all's good.
What matters is surface area contact between the steerer and stem.
Some stems have a minimal contact area at the front which isn't really suitable for carbon steerer losing that couple of mm makes it worse. Even with a steel steerer it can be a problem with the stem rocking if it doesn't have a proper contact ring at the top but it is pretty easy to see what contact area the stem has left on the steerer or if it rocks with top cap off and bolts loose.