Tranny Mainshaft Circlip Install

Textf you ride an airhead BMW made between 1984 and 1995 your transmission may be on its way to lunch. That's because sometime in 1984, BMW decided to stop installing a circlip on the transmission main shaft. The circlip keeps the bearing next to it from moving axially on the shaft, putting axial loads on said bearing. The walking is induced by axial loads imparted to the shaft from the helically cut gear that drives the shaft from the input shaft of the transmission. Ball bearings really don't like axial loads, and often show their displeasure by shedding the cage that keeps the balls separated. When the cage pieces fall out of the bearing inside the tranny they can become lodged in other bearings or between gear teeth, and the tranny quickly quits transmitting power. Frequently it locks up, leaving the rider skidding down the highway and stranded at the side of the road. In the winter of 2006 I started looking forward to a Spring trip to Baja, Mexico, and the thought of this ticking time bomb inside my bike started to really chew on my conscience. I'd never seen anything other than fine flour deposits on the tranny magnetic drain plug during oil changes, so I was pretty sure there wasn't anything drastically wrong with the box. Even though some bikes made in 1993 did have circlips (that is the transition year when BMW came to its senses and started re-installing the circlips, possibly starting with gearbox number 240765) I knew this one didn't because I'd seen it missing when Dave Gardner upgraded first and fifth gear a few years before. He didn't have the equipment to machine a new groove into the mainshaft, and said it was nothing to worry about anyway as he'd never seen a bike with the problem. Never the less visions of being stranded along some desolate Baja dirt track motivated me to start looking for some help in getting the potential problem defused. Ted Porter works out of his BeemerShop in Scotts Valley, California, a mere sixty or so miles from my home, and it didn't take long for an email inquiry about the job to be answered in a very positive and professional manner. I pulled the transmission out of the bike (only took an hour or so) later that week and had it down to Ted that Saturday.

The BeemerShop is located in a small industrial park, and one of Ted's neighbors has a big honkin' lathe that he lets Ted use as needed. Ted has worked out a tool profile and holder that he uses only for cutting mainshaft circlip grooves, and uses a dial indicator on the lathe carriage (the white dial in the lower center of the image above) to locate the groove.

Obviously the transmission must be disassembled to access the mainshaft, and while it's apart Ted gives everything a careful going over to make sure there are no problems. In the case of this transmission he found that the bearing next to the new circlip groove was rough and had a fair amount of play between inner and outer race, so that was replaced. That was consistent with the resistance we felt when we tried to turn the output flange before disassembly. With no other damage evident, it looked like my caution had been rewarded.

The groove isn't very deep, and being located almost at the end of the shaft there's little danger of it being the nucleation site for a crack (lost a layshaft in a Norton transmission once that way). With the bearing assembled onto the shaft, there's often enough room to let the bearing move a bit, and in those cases Ted installers a small wire ring between the bearing and circlip. In this case there wasn't sufficient room for the ring, probably due to differences in part tolerances and placement of the groove. Ted is confident that not using the ring doesn't compromise the ability of the circlip to keep the bearing from moving. Ted charges $50 for machining the groove and another couple hundred for stripping the box, cleaning everything, and reassembling. I found that the rear main seal of the motor had been leaking and picked up a replacement from Ted when I dropped the transmission off. My total bill, including the new cover gasket and new bearing, was about $335, a pittance compared to the cost of recovering a bike from the depths of the Baja.

Photographs courtesy of Ted Porter.