From the WW I era to the 1970s, U.S. made bicycles, often of very high quality, were built to rather different standards than those that prevailed in the rest of the world. These bicycles used different standards for bottom brackets and cranks, frame tube diameters, seatposts, pedals, headsets and handlebar stems. When the 1970's bike boom brought large numbers of European (and later, Asian) bikes to the U.S., the older American dimensional tolerances went into retreat, and were increasingly relegated to department store bicycles, such as Huffy and Murray. People began to associate the American dimensions with these low-end bikes, and to forget that many high-quality machines had been made to these standards. While the European bikes were undeniably lighter and faster, the better American bikes were made with a greater emphasis on durability and ruggedness.
BMX bikes are still largely made to American dimensions, and the BMX segment is one of the key sources for American sized parts.
The early ancestors of the mountain bike were based on older American "cruiser" frames, and many early purpose-built mountain bikes used these standards.
American-style bikes have thicker steerer tubes than Euro bikes, so they take thinner stems. The usual diameter is 21.15 mm vs the British 22.2 mm size. The stems listed below are this size. Sorry we don't have more to choose from, but these are getting scarce.
The handlebar clamp diameter of American stems is usually the ISO standard size, 1"/25.4 mm, or, in BMX applications, 7/8"/22.2 mm, so matching bars to stems is not commonly a problem, as long as the stem fits the steerer.
Alloy/Steel Stem SM242 $24.95 
These were unavailable for a long time, but we found a new stash!Ideal upgrade for your Schwinn Varsity or other older bike that needs the 21.15 quill size.
Aluminum upper section to keep the weight down, with strong steel quill.
| Forward Exension (reach) 3" (80 mm) 8" (200 mm) tall. (bottom to center of bar clamp.) 440 grams Clamp fits ISO standard 1" (25.4 mm) handlebar center. |
"Swan"-Type High-Rise Stem SM245 $19.95
These are all-steel with a short 65 mm extension. These are heavy, but if you want your handlebars really high, this is what you need.
| Forward Exension (reach) 2 1/2" (65 mm) Quill is 10" (254 mm ) tall. 500 grams Clamp fits ISO standard 1" (25.4 mm) handlebar center. |
21.15 mm Stem Raisers SM247 $19.95
A stem raiser fits into the steering column in place of your present stem, and secures with a wedge bolt in the normal manner. The upper part of the stem raiser sticks out above the steerer, and is bulged out so that you can insert your original stem. The bottom of the original stem winds up a bit above the top of the headset, so you gain several inches of height.
Note:
Steerers are butted at the bottom, so the hole in the steerer is constant-diameter until near the bottom, then the walls taper inward in the butted section. It is vitally important that the stem extender (or stem) is not inserted so far that the wedge is installed where the steerer is narrowing, or it could come loose unpredictably. This is sometimes a problem on smaller frames if you try to insert the stem or a stem riser too far down into the steerer.
In addition to the option shown above, there is an older type of conversion kit that includes the bottom bracket. These are generally less satisfactory, and have become pretty much obsolete. The models commonly available have spindles that are longer than most newer crank sets are designed for, so the cranks may be uncomfortably far apart, with poor chain line.
