We left off last post with the new down tube mitered at the bottom bracket end. As you are about to see, you really have to do this miter first since you won’t be able to clamp the tube once the coupler is installed. To maximize the amount of the coupler in the butting section I install it as close to the bottom bracket as possible. There are a couple of things you need to take into consideration here though; on larger frames you need to place the couplers in a spot so that the halves of the frame fit into the box and with lugged frames you need to make sure there isn’t any interference with the lug. This particular frame has a 54cm seat tube, so the frame will still fit into the S&S travel case with the couplers near the end of the tubes. As you know, this frame is a TIG welded frame, so I didn’t need to think about lug interference. In the picture below you can see how close to the bottom bracket miter I position the coupler. Also in the picture below you can see that the half of the couple closest to the seat tube (on the top tube and the down tube) is the threaded half…this is important because if you put the other side on you won’t be able to get the nut unscrewed enough to seperate the frame!
A slight change in the perspective of the picture shows the seat on the inside of the coupler and the dash I made of where to cut the down tube. When the resulting stub is inserted into the cupler the tip will be about 1/4″ away from the bottom bracket miter.
I like to use a simple hacksaw to cut my tubes to length. An abrasive chop saw was gifted to me, but I think the hacksaw is cleaner, quieter and inputs less heat into the tube. Also, a hacksaw cut is pretty darn quick!
After the hacksaw cut you’re left with a stub of a tube.
The down tube stub is then dry fit to the bottom bracket/seat tube. The tubes below look a little different because of the retrofit process. I don’t do a lot of work here because you don’t really know the final fit without the rest of the down tube on there and dry fit to the head tube.
The picture below shows the coupler that will be brazed to the down tube stub that we just created. You can see the stainless steel disc that is supplied with the couplers. This is actually a seperate piece from the coupler itself and will create a water-tight seal when properly brazed. The coupler is stainless steel. Stainless steels love to oxidize with ambient air, so right before brazing a thin layer needs to be ground/filed/sanded off the inside of the coupler.
The down tube is cleaned very throughly to ensure a strong brazing bond with the coupler. The stainless steel starts to oxidize pretty quickly, so after the inside film is removed from the coupler I flux the enitre piece and mate the coupler to the down tube stub. It is important to orient the coupler to the stub in the right rotational phase. You really have to go by your eye here, but you’d be surprised how close your eye can measure things like this.
This last picture from this post shows the stainless steel disc inside coupler and how everything is fluxed. Flux keeps the tubes from oxidizing while brazing and keeps everything very clean at the elevated brazing temperatures.
Unfortunately this will be my last post of this for a few days. I am still on business in central Manitoba, Canada.