Step 19: Bonding on the Brake Bosses
You can buy these parts from a frame supplier, but in my case I liberated them from the same donor frame that provided my headtube and dropouts. I retained some of the tube that the brake bosses had been welded to, and then ground off the back half of the tube. I then used a half-round file that approximately matched the diameter of my seatstays and filed the interior of the tube.
I mocked up the pair of bosses on my frame and measured the spacing between them. I then drilled two holes with the same spacing in piece of scrap wood. Plugging the bosses into these holes ensured that they would be properly aligned.
I then used some high quality kneadable metal epoxy putty and put a layer on the back of both of the bosses. I then pressed these against my seatstays, pressing hard to squeeze out any excess putty but allowing it to fill any gaps between the boss and the seatstay, creating a solid bond. Make sure that proper alignment is maintained, because the metal epoxy sets up quickly (usually 10 minutes).
After the epoxy had set, I sanded away the excess and create a lashed joint with some carbon fiber tow (thin unidirectional ribbon that comes on a spool) for extra strength and insurance. I first wrapped the joint in fiberglass, then wrapped several Figure-8s with the carbon fiber until it was uniform and covered all the edges of the boss. I then wrapped it in electrical tape to compress it, as before.