Originally Posted by markjenn

It would help to know more about the specifics - all stems are not created equal and the technology has changed pretty dramatically in the last 15 years or so with the general replacement of quill stems with threadless. A single picture would help.

A couple things to keep in mind that might help you scope the job:

1. For a threadless stem, the stem relies on friction between the steering tube and the stem for steering control. This makes if very important that the clamping surfaces are clean and free of any lubricant. It also makes the torque of the clamping bolts pretty important. If you don't have a torque wrench or a good feel for this sort of thing, you could overtorque the bolts to where they could suddenly fail or you could undertorque them such that they do not have not have sufficient clamping pressure to maintain steering control in an emergency situation.

2. Also on a threadless system, the preload of the bearings in the steering head depend on the torque of the top bolt that pushes the stem down against the steering head. And this torque needs to be set with the stem clamping bolts loose so the stem can move vertically with respect to the steering head. So the sequence of tightening (top bolt first, then clamping bolts) is critical.

3. If you have a closed clamp where the handlebar attaches to the stem (almost a complete circle around the bar), then you are going to have to completely disassemble everything on at least one side of the handlebar to be able to get the bar and stem separated (e.g., tape, levers, shifters, etc.). This is not difficult, but it is time consuming. If the handlebar clamp has two bolts which removes half the clamp, things are much easier. Same caution about clamping bolt torque here as above.

4. If the new stem has a different stack height from the old (the part of the stem that overlaps the steering tube), you could have insufficient steering tube length to get proper clamping or you could have too much length such that you need to add or shorten any spacers you might have. There are guidelines and tolerances for these dimensions.

This isn't terribly difficult stuff, but it needs to be understood and the consequences of really screwing it up are pretty serious.

- Mark