Alternator - sheered bolt and squashed cush rubbers

My Daytona had been rattling for ages from beneath the carbs. I'd noticed it only did this when the alternator was not under load. As soon as I switched the lights on, the rattle went away. Amazing but true. The light switch was a rattle controller! 

Reading around, I'd found people with this problem saying it was to do with a worn alternator cush drive. On a visit to Sprint, I talked to Trevor about it and he said it was far more likely to be a problem with a bolt that holds a rotor onto the alternator itself. It turned out to be both ...

The pictures above show the bolt loose on the driven rotor (23 in the schematic) and the replacement bolt next to the remains of the original to show how much was left in the alternator rotor. The drive and driven rotors have four vanes the fit into one another, cushioned by eight rubber segments as the 'cush drive' (number 20 in the schematic below).

Parts schematic for alternator drive gears

 The pics below show the worn cush drive rubbers in the drive rotor and comparing them to the new replacements.

I'd also had some trouble with the starter motor so pulled that off at the same time as removing the alternator. This meant draining the coolant because the rear coolant hose runs from the back of the cylinder block to the pump and obscures the starter.

Draining the 900's coolant - looking grubby ...

... but the black finish always comes up really well with degreaser and a bit of gentle agitation

The bolt that retains the driven rotor is 'encapsulated', meaning it comes coated with a vibration-resistant coating. As a result, it was hard work to get the remains of the original bolt out. I had to drill it right through. The main trick is getting the drill bit central in the remains of the old one and holding the work still. I don't have a pillar drill but a Record drill stand and press vice. The alternator is a very awkward shape all the same so I had to put a lot of packing around it to get the press vice to grip.

Drilling out broken M6 bolt with a 5mm HSS bit

After drilling and cleaning threads with a tap

 I'd done a fair job but the internal threads were clipped by my drill. Fortunately the encapsulation is meaty enough for me not to worry about that too much. I was able to torque it up carefully to the recommended level so all looks well.

Torquing up the replacement bolt, holding the driven rotor with an adjustable spanner

The replacement cush rubbers make for a close fit between the drive and driven rotor. I held them into the drive rotor with red rubber grease. It was a fiddle to line up the vanes on the driven rotor to get the alternator back in place but eventually went in with zero slop.

Mission accomplished.