Restorers of early racebikes generally want to put everything back to stock, as it came out of the crate. As an example, when riders took their new disc-braked Yamaha TZ750As to Daytona in 1974, nearly all of them switched the calipers from the front of the fork legs to the back, and that’s where they stayed. But restorers, seeing those calipers on the front in the parts book, put them back where riders found they made the steering less stable. Riders generally get their way; you don’t see many late-model bikes with calipers ahead of the fork legs.

Many a restorer of the Yamaha TD1 series (TD1, then A, B, and C variants) has found a non-stock rear brake torque arm on it, handmade from aluminum. And it's not easy to find an original 143-prefix steel arm to replace it, even though the original price was $3.20. Here's why.

Boston-area rider Frank Camillieri rode for Boston Cycles, and in the course of events took delivery of a new 1967 250 TD1-C roadracer. Off he went to that year’s series of races in the Northeastern US and Canada. During one of his races, he braked for a turn ahead and heard a loud bang (“It was like a shotgun going off, right behind me!” he said). When he rolled into the paddock he found that the steel rear brake arm had broken, allowing the brake backing plate and shoes to whirl around with the rear wheel, winding the hefty steel rear brake cable rapidly around the rear axle until, having become taut, it broke.

Frank Camillieri
Boston-based privateer Frank Camillieri is seen here—50 years ago—lapping the Loudon, New Hampshire, road course on a Yamaha TD1-B.Michael Green

Looking at the broken part it was clear what had happened. Each end of the torque arm widened to provide strength around a bolt hole. To further reinforce those mounting-bolt holes, each end of the arm had a reinforcing plate resistance-welded to it. The failure had originated at one of those welds.

Rider conversations soon revealed that others had had the same shotgun experience, so it became standard practice to make a new rear brake torque arm of aluminum, putting an end to the failures. Although this no doubt annoys perfectionist restorers, this was how many of those bikes were raced.

Why would the steel arm break? In resistance welding, the two pieces to be joined are clamped together by a sort of copper tongs, and the electrical charge on a great big capacitor is dumped through them. Because there is high resistance between the two steel parts being joined (the torque arm and one of its reinforcing plates) heat is generated in proportion to the electrical resistance times the square of the current. In an instant, the metal melts enough to fuse and in an equally short time it is suddenly cooled by the mass of metal surrounding it.

Could large stress be generated by the sudden expansion and cooling of the weld? Was the state of the metal’s temper changed by this heating, possibly making it brittle? Was this further complicated by chrome plating the finished part, a practice known to bring a risk of hydrogen embrittlement? Lacking the means to investigate, racers just put that part on their mental “replace-before-riding” list, along with the plastic bushings in the swingarm (which appeared to have been molded from crushed peach pits), the original tires (which riders dismissed as “rim protectors”), and the shocks.