The stem is a metal rod inside your mountain bike forks that you adjust vertically to change the height of your handlebars. It also attaches to the steerer above the wheel so your front wheel turns when you move the handlebars. The size of the stem must fit both the steerer and the handlebars to operate properly.
Stem Diameter
You measure the diameter of your mountain bike stem based on the diameter of the forks. To find the proper stem size, subtract 1/8 inch from the diameter of the forks. According to bicycle mechanic Sheldon Brown, a majority of mountain bikes have forks that require a 1-inch stem diameter.
Stem Length
You can adjust the stem length on your mountain bike, so measurement of this dimension is determined by where you want your handlebars. According to author Steve Worland in "The Mountain Bike Book," bikes with a longer stem shift your weight over the handlebars to make your steering more stable. If you opt for a shorter stem length, your weight will reside more over the seat post, which allows you to turn easier but decreases your stability.
Handlebar Compatibility
At one point, bicycle manufacturers produced a variety of handlebars to fit many different stems. This made it difficult to mix and match stems to different headsets. Brown says modern bikes are fitted with only one of two stem sizes at the handlebars. Mountain bikes have upright handlebars and a stem that is 7/8 inch where it connects to the handlebars. Because this is a fairly universal measurement, you are unlikely to need to manually measure the top of the stem.
Stem Extender
If you need to replace the stem because you want to raise your handlebars, you may be able to achieve the same effect with a stem extender. The extender fits into the wedge inside the steering mechanism and can raise the stem about 2 1/2 inches higher than its standard level.
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