Chapter Five - Building the Sub-Assemblies

STEP V-5: The next joint to do is the seat tube into the bottom bracket. This joint is also massive and requires a long preheat. All of the brazing procedures are the same with this piece as they were with the fork crown/steering column. After the joint has reached the right temperature (it is not necessary for the whole joint to be the right temperature, just the immediate area you are working on.), the silver can be dropped along the outer rim of the seat tube socket of the bottom bracket shell. Figure 5-11 shows the bottom bracket being brazed.

FIGURE 5-11: Brazing bottom bracket/seat tube

FIGURE 5-12: Pointing the flame inside the bottom bracket shell

 On this joint too, the silver should be added at one location and pulled through till it can be seen in another location. Figure 5-12 shows how the flame is inserted inside the bottom bracket shell to pull the silver solder the rest of the way through the joint. Be careful when working inside the bottom bracket shell, you can be overheating an area without even knowing it. Another problem can develop while brazing inside an enclosed area; unburned acetylene can build up in the bottom of the seat tube opening and suddenly ignite causing the flame to extinguish. Gently blowing your breath into the bottom backet shell while brazing in this enclosed area will move the unburned gasses out of the way and allow the flame to continue burning. Be careful not to inhale any fumes while doing this.

While heating inside the bottom bracket shell, use the flame to inspect to se if silver has been able to penetrate all the way through the joint.

There are particular problem areas in the bottom bracket shell. It is somewhat difficult to pull silver down to the tubing points (right between the two parts of the compound miter) and a little extra heat in this area will probably be necessary. It is also hard to pull silver straight down between the chainstay sockets along the tube. This area is sometimes an area of frame failure; this part of the joint can be completely dry of silver without being evident on the outer surface. It can eventually rust between the tube and the bottom bracket shell which can lead to cracking or even joint separation. Apply a little extra heat between the chainstay sockets to make sure the silver is drawn down through this area. Figure 5-13 shows heat being applied between the chainstay sockets.

After you think the joint has been finished, go around the outer rim of the seat tube socket with the flame and look for gaps. If any gaps are found, draw more silver into them with the flame. If more silver is necessary to do this, add some. If gravity will help, turn the piece up side down.

After the joint has been inspected for gaps, let it cool for a few minutes before taking it out of the fixture. Taking a hot piece out of the fixture is not the best idea; A hot piece has not had a chance to stabilize and may still continue to distort during the cooling process.

Set the seat tube/bottom bracket shell aside with the fork crown/steering column. We'll come back to them later.

FIGURE 5-13: Applying heat between the chainstay sockets

FIGURE 5-14: Brazing dropouts into fork blades in a vertical position

Place your order for the Paterek Manual.
To choose your country,
click here.
E-mail Tim Paterek - click here.