Below is a picture of my cantilever boss fixture.

The crossbars on mine are made of angle iron from an old bed frame; the vertical pieces are 3/4" (19 mm) X 1/8" (3 mm) metal stock. The central vertical piece is rigidly bolted to the two crossbars, perpendicular to them.

The short vertical arms are attached to the ends of the upper crossbar by single bolts which allow them to be pivoted in and out, to accomodate different frame widths. These bolts get tightened securely to hold the arms at the correct spacing.

At the top of each of the moveable arms is a 1/4" (6 mm) hole. A 6 mm bolt goes through each of these, and screws into the end of the cantilever boss. The lower crossbar rests against the stays or fork blades. A long bolt pulls up on a separate crossbar below the stays, to hold the fixture in place during brazing.

The cheaper, old style studs are two-piece, and do need to be brazed together. Modern one-piece studs cost a tiny bit more, and are well worth it.

To check for the ideal position of the bosses, I start by bolting one cantilever to a loose boss, and holding it against the stay (with a wheel installed) to see where it lines up best. I then measure down from some reference point, such as the seat lug or the bridge to the top of the boss where it attaches to the stay. This is better than trying to mark the spot on the stay itself, because the stay needs to be cleaned down to bare metal where the boss is to be attached.

Due to the adjustability of cantilever brakes, several possible positions may be usable for the brazed-on studs in any given application.

In some cases, it may be desirable to use a compromise mounting position for a bike that might be used with two different-size rims. In most cases, you will want to optimize for one particular rim size.

If the stud is mounted closer to the rim, so that the shoes must be set to the lower limit of their vertical position, mechanical advantage (power) will be greater than if they are farther away, so that the shoes need to be set farther away from the pivot point (the stud.)

The other major concern is the direction of travel of the brake shoe as it strikes the rim. The shoe moves in a circular arc about the stud. The farther inboard the stud is mounted, the more nearly horizontal the shoe's motion will be as it comes toward the rim.

If the studs are too far outboard, especially if narrow rims are being used, the shoes will be moving downward as much as inward when they strike the rim. This can create two possible problems:

1. As the brake shoe wears, it may undershoot the rim and get caught in the spokes.
2. If a wide tire is used on a narrow rim, the risk of the brake shoe grazing the sidewall of the tire is increased.

If you are in doubt of your skill as a brazer, I'd suggest that you not try attaching bosses to your front fork, because the results of failure on the front can be quite cataclysmic. Besides, you can buy a fork with ready-installed bosses to fit most applications.

Boss installation on the seat stays is a much less risky endeavor; even if you overheat the joint and weaken the stays, a failure here is unlikely to represent a severe hazard.