Cannonball: Broken crank
I was on Stage 1, day 1, mile 70 of 3300, riding at 1/2 throttle, going about 30mph, then a sudden bang! bang! bang! from the bottom of the motor. Clutch in, exhaust valve lifted, I stopped on the side of the road. While I waited for a chase truck (who was stopped somewhere behind me to pick up some other broken bikes), I started to tear down the motor to see what had happened. By spinning the rear wheel, the V belt spun the crank pulley and the left flywheel. The motor would turn over by hand, and the piston was seen to rise and fall after I removed the valve cover. Valves went up and down as the right side flywheel spun, but there were very audible thump sounds at TDC and BDC… When I grabbed the hot pulley in my hand, I figured it out. The pulley could flop slightly from side to side. The crank pin was broken, but it still made enough contact to spin one flywheel when the other was spun. I couldn’t get the motor out of the frame right then, as the truck and trailer honked at their arrival.
Loaded, then the long slow drive to the hotel, punctuated by stops to get 2 more broken bikes… Plenty of time to sit and think and wish that I was riding and not in the truck on the first day of a 17 day rally. I didn’t have any comfort in knowing that 5 or 6 bikes came in on trucks, or knowing that other motors blew up even worse than mine. The crank pin was 97 years old, and had done a few thousand miles for me in the last 5-6 years. But it was time for a new one and I didn’t have a spare in my tool kit. No damn way was I going to pack up and head for home.
Two friendly motorcycle policemen came by the hotel parking lot around 11pm. While I was tearing into the motor, some of the guys told them about the crank. Next thing I knew, one of the officers handed me his cell phone and said “here’s a local machinist”!!! I was elated to hear that somebody was available, but felt a little bad that he was awakened by a call from the police at 11:30 on a Friday night. After a quick chat, we decided to meet at the shop at 7am Saturday morning to make some new parts. He did make of point of asking if I was a member of the AMCA. Sometimes it pays to be a member.
Jeff and Tommy at S&S Repair came to the rescue. The shop does general fabrication and repairs for all types of things, and understands motors. In this photo Jeff is deburring the new crankpin on the big 16” lathe.
Broken crank pin and new crank pin. The old one has bronze deposited on the bearing area, from the big end bushing in the connecting rod. The bushing had spun in the rod, which covered up the holes that allowed oil to splash and drip onto the pin. The pin seized in the bushing, then the pin sheared.
Big lathe, about 20”
Bigger lathe. Around 30”. That round part is the flywheel for my motor.
REALLY BIG LATHE. About 10 feet in diameter. This is used to turn parts vertically. I’m standing at the center of the spindle axis. S&S uses this to turn parts that weigh several tons. They also have a 400 ton hydraulic press out back.
Part 2 will come later. Hopefully I’ll write that the motor came together OK and that I was ready to ride on Sunday for Stage #3.