Sunday, January 29, 2012

Mike's R90S Gearbox

My good friend Mike is one of the first people to introduce me to motorcycles. As far as I can remember he lived by his old Norton for over 13 years or so. I met him around the time that the old Snort moved on. He's a one motorcycle type of guy and is not afraid of using them, quite the care taker if you will. Think hard on being loyal to an old British machine for a decade plus...

Somewhere around 12 or so years ago he became the next care taker of an BMW R90S. Mike's 'S' gets ridden whereas others are hidden. Always on top of maintenance mike has not let the 'S' down either. So a few weeks back Mike thought it might not be a bad idea to go into the gearbox for an inspection while it was out of the frame during his seasonal spline lube. It also had a broken grounding boss near the speedo that needed to be welded up while the cover is off.

Here are a some pictures of the gearbox and its internals. The process is quite simple to open up a BMW gearbox but it helps to have two special tools; one to hold the output flange for removal and the other to remove the cover. Cycleworks makes a many in one tool to help with the flange removal.

BMW uses an interference fit between the bearings and the cover so a heat gun gets things good and hot before removal. The tool fitted in the throwout bearing location is tightened up and helps push the cover up off the bearings. A little light tapping helps free things up. If you don't have the tool you can use a section of hardwood and tap the cover off at three points of additional castings on the edges, never pry- you won't like the results.We found signs of bearing degradation; flakes of metal from the bearing races. Having never been opened for over 30 years and getting close to the 100,000 mile mark, it would be getting time. One of the old timers' that has since passed said you could set your watch by some of the bearings replacement intervals; he had over 680,000 miles on his 60/5.Two bolts on the clutch side of the box hold the shift selector unit in the case. this picture shows the selector unit extracted along with the input shaft.Mike and Duncan examine the freshly extracted gear selector unit. Being untouched since it left the factory; the shift selector pawl spring was old the old style design. The factory but two extra bends on these to help it index against the pawl lever. As time goes on these bends fatigue, crack then break leaving you in whatever gear you last selected. Best to change it whether you think it needs to be done or not.
Once the input shaft and selector unit are removed, the gearbox case can be heated on the clutch side to remove the lay and output shaft.
We'll be replacing 4 of the 6 bearings, the shift pawl spring all seals and cover gasket As the job moves forward.