I need to replace a stem but I'm not sure how to figure out what size the stem is. I tried and failed to understand sheldon's instructions. I might be reading my caliper wrong but I thought I came up with 18.5 mm in diameter for the stem.

Thx

What part are you measuring? Quill or the newer style?

quill. I think I figured it out, I was reading my caliper wrong

Quill stem steerer tube interface
Measure the outside diameter of the insertable part to determine fork compatibility. The (British) standard for what is called a "one inch quill stem" is an insertable diameter of 22.2mm. Note that this is not 1" (25.4mm) because the 1" in the name refers to the outside diameter of the steerer tube, and the stem fits within the inside diameter.

There is also a 1-1/8" quill stem standard, which is (I believe) about 25.4mm insertable diameter. These are mainly found on older mountain bikes from before the time that threadless systems took over.

Note that some older and weirder bikes appear to use the 22.2mm standard, but are actually slightly smaller. The most common one I see is Schwinn forks, which require a 21.15mm insertable diameter. A 22.2 stem will probably fit if pounded in with a hammer, but it definitely aint right! Likewise, a 21.15 stem will fit into, but feel really wiggly in a fork designed for 22.2mm. Double check your measurements of both fork and stem if you aren't sure what you're working with.

Also, always make sure that the minimum insertion line is below the top of the steerer tube. Raising a quill stem too far can cause the stem and bars to suddenly detach from the rest of the bike...

Threadless stem steerer tube interface
There are only a few common sizes of steerer tubes (and correspondingly stems) in the threadless system, and they are pretty easy to tell apart. The most common is 1-1/8", which is the approximate outside diameter of the steerer tube and the approximate inside diameter of the (loosened) clamp on the stem.

Other uncommon sizes include 1", 1-1/4", and 1.5". These are all straight forward; measure the OD of the steerer tube or the ID of the stem and it should be obvious which size is closest.

Clamp diameters for quill and threadless
25.4mm (1") is the most common for a lot of mountain and flat-bar configurations. It's beginning to be far less common in favor of newer standards, but it is still out there. It is very rare to find road (drop) bars with a 25.4 clamp diameter. and it is not very common to find flat bars or cruiser bars that are not 25.4mm.

26.0mm is (well, was) the standard for road bars for quite some time. It's close to 25.4 but NOT close enough at all! Special shims are made to clamp 25.4mm bars in 26.0mm stems. Trying to fit a 26.0 bar into a 25.4 stem is also a BAD idea. Don't try it!

The new kid on the block is 31.8mm, commonly called "oversized". It gives you a stiffer connection and is less apt to twist in the clamp due the the increased clamping area. 31.8mm is used on the majority of higher-end (and even mid-end) mountain and road bikes. There are shims to use a 31.8 stem with both 25.4 and 26.0 bars (and you need to make sure you have the right ones!)

Measuring a stem's height, angle, and reach
The same basic measurement norms apply to both threadless and quill stems. The most common mistake people make is to measure the reach of the stem along the angled portion. The correct method is to measure along an imaginary line perpendicular to the steerer tube.

It comes up a lot less often, but there are also right and wrong ways to measure the height of a stem. I don't remember them offhand, but it would make sense to be measuring the distance the center of the bar clamp sits above the top of the headset. Measure parallel to the steerer tube, not at some angle.

Ted,

I have a number of extra stems. Before you go spending money, we should talk.

It's an old brit Raleigh stem -