Stems, spacers, and threadless headset tolerances
 

I just replaced the original stem on my Surly Cross-Check with a Salsa Moto-Ace SUL stem with 25.4 clamp size so that I could get a higher rise (trying out moustache bars). Fork is 1 1/8 threadless.

I didn't realize ahead of time that the stack height on the Salsa stem would be taller than on the original stem. Problem is, I had three 5mm spacers on the steerer in the old configuration. If I remove all of them, the top edge of the new stem is just barely higher (less than 1mm) than the top edge of the steerer tube. If I only remove two of them, there's a hair more than 5mm between the top of the steerer and the top of the stem.

The literature I've read on threadless stem installation and headset adjustment says that you should have a 3mm gap between the top of the steerer and top of the stem in order to insure that the bolt on the headset cap is securely threaded into the starfangled nut. My question is, how close is this tolerance? I've completed the installation with one spacer installed, and it seems sturdy enough, but is this a disaster waiting to happen?

Obviously, the solution is simply to find a 2mm spacer, but I'd rather not go shopping again if I don't have to. But I will if it means not breaking my neck (perhaps I just answered my own question...)

Thanks!

5mm between top of steerer and top of stem is not an issue.

If you want to split hairs, it depends on the stack height of the stem, and the location of the clamp bolts. A tall stem with widely spaced clamping bolts should not be a problem. A stem with a low stack height and a single bolt may be cause for concern. Look at it as the percentage of unused stem clamping surface.

I agree. Actually, I've gone even more than that, but only with stems with tall clamp height, making sure the steerer tube cleared all the pinch bolts.

How low is low? Even a 30 stack stem would still be clamping 25mm. The one he used is a 52 2 bolt.

I had a titanium stem with about 30mm of stack height, and a single bolt clamp. If it was on a road bike, and only weighed 50kg, it would not be so much of a concern. But it's not, and I don't, so it is.

Thanks for the responses. So the idea is that a tall stack height stem with two pinch bolts is doing a sufficient job securing the headset to not be too concerned about the tolerance on the cap bolt going into the starfangled nut?

You still need to be sure that you've got enough thread into the star nut to snug things up, but the nut itself serves no purpose safety-wise.