I need a stem longer than the one I have. How do I determine what length I have so that I know what length to order? Also, mine measures 22mm in diameter in the section that is above the steering head. I've seen advertised stems of 22.2mm, but not 22.0. Where is the measurement taken? The bike is a 1987 Schwinn Prelude.

Stem length, or more properly, forward extension is measured from center to center along the extension itself, which is usually horizontal. I usually measure from the center of the expander bolt head to the center of the handlebar clamp section as best as I can judge by eye. You don't need super precision as stems come in standard sizes in expressed in millimeters, ending in either zero or five, ie. 90, 100, 110, etc, or 85, 95, 105, etc. so you shouldn't have a problem.

Stem diameters are usually 7/8" or 22.2mm, and your Schwinn should be the same. The 22mm size is almost exclusively used of French bikes whose forks are 25mm in diameter vs. the common 1" or 25.4mm diameter.

Double check your measurement before buying the stem, since if too large it simply won't fit, and if too small will be sloppy and wobbly.

Probably if you take the stem out the headset you will see the numbers indicating the sizing also (at least the good stuff has the numbers stamped)

On standard 73* quill stems most manufacturers just measure across the extension center to center from the stem bolt to the middle of the clamp.

Cinelli, on the other hand, measures perpendicular to the quill - resulting in an "measurement" that's indicated about 5mm shorter than as measured by other brands.

Your Schwinn should use a standard 22.2mm. You need to match the clamp size to your handlebar. Most [used w/ quill] are 25.4mm (1") or 26.0mm, but many Cinelli are 26.4mm. Don't try to use the wrong size. You can often make it work, but why?

I was unclear in my explanation of the issue. I need a stem that is taller than the current one. The forward extension is fine. There is a limit line that says "Max" on the vertical part current one that is about 1/2" above the steering head when the bars are in a comfortable position. I would like to get one that keeps the handlebars at the same vertical height and results in more engagement in the steering tube.

There's always the Technomic. Make sure it isn't too tall for your needs, unless you're OK with cutting it.

An old Schwinn probably has a 21.15mm stem (.833 inches), right? Unique dimension to American bikes.

The Nitto Technomic would still work but you need to put it on a lathe and turn it down from 22.2mm to 21.15.

I don't think the 87 Prelude had .833 stems, that is something to consider, however.