I'm busy building up an old road bike and want to have the bars raised up a bit, but the quill stem is short and angled downwards. I have another stem off a Walmart style MTB but it is slightly thinner where it goes into the steerer tube. The angled wedge arrangement is pretty simple so it does tighten up nicely, but I'm wondering if it will do any damage to the steerer tube? Thanks

Start speaking in measured diameters and lengths for better clear communication, here.

Don't take chances with stems or forks; get a part that fits properly. Failure of a stem or fork can be highly unpleasant.

A couple options if you're looking to get your handlebars higher: 1) Nitto "Technomic" quill stem



2) a threadless sttem adapter and modern riser stem:

+100 Having the bars come off in your hands is only funny in the cartoons.

How much thinner, in mm? If the walmart stem is steel then it will be plenty strong.

The problem with using a stem too small in diameter to best fit the steerer's ID isn't when first assembled. It's when a year or more has passed and that area of the bike hasn't been kept on top of. Stuff looseness, metal creeps when not loaded as intended. People tend to get complacent. As things move a bit more, but not enough to feel unless looking for it, the effects of ill fitted parts produce secondary concerns. Like the steerer becoming ovalized from the wedge. If that wedge was also placed so it overlapped the threads then all those stress risers might show why threads are stress risers. Andy

There are BMX style stems that are smaller than road bike stems, and are not compatible.

Most MTB stems will either fit a road bike, or will be much too big.

Angled up or down doesn't make much difference, and is all user preference, although traditionally road bike stems angle downward (or parallel to the road considering the headtube angle.

there is smaller stems than 22.2mm_7/8"
13/16" _ 21,15 mm & French 22.0mm..
this is why measuring is valuable.

Thanks for the replies everyone, and apologies for being lazy. The 2 stems are 22mm and 20.7mm (as best as my cheapy vernier will measure). Yes the smaller one is steel so strength is not an issue, just damage to the steerer tube as Andrew R Stewart pointed out. I will stick with the original stem for now and see what comes up later. I have access to a lathe so the threadless stem adapter is also an option. Thanks again.