When you pull the starter cover, it should be pretty easy to tell if the starter motor is somehow engaged with the flywheel and binding up the motor. Otherwise I would guess that there has been a catastrophic engine failure. Also, 11.79 battery voltage after charging is a little bit low.

I pulled the cover and found a catastrophic failure of the starter casting. I posted some photos here: https://goo.gl/photos/sNX74Uv9rr5LnAoM9

The vendor got back to me today and insists that the failure is the result of low battery voltage. My two or three year old AGM battery was only reading 11.79 V after charging. I still find it hard to believe that a low battery caused the failure. I've had lots of airheads with failing or discharged batteries that wouldn't crank the engine and none of those bikes' starter "grenaded" this way. The vendor claims that low voltage led to ignition malfunction and backfiring which destroyed the casting. We didn't hear any backfiring. Just an engine that wouldn't start followed eventually by clicking. Anyway, the damage is done. Time to find a new starter. The vendor has kindly agreed to replace the starter after making it clear that the damage is not covered under warranty (which is appreciated if not completely understood by me). However I'm not sure I will install the replacement unit. I might try getting the original Bosch unit rebuilt or buying up the replacement to a genuine Valeo (apparently the same as the current OEM part).

Very frustrating especially considering that I may have to pull the transmission to check for debris that could damage the clutch. Whatever I saved by buying this starter to replace the worn out Bosch (approx. $300 price difference) will now be consumed with hours of disassembly and reassembly. Good thing I don't charge for my time. 🙁 On the positive side I'm going to be much more familiar with starter motor replacement than I was before.

Ride safe,

-Gary

I saw this once before on an R90/6. In that case, the owner had the timing set to an extreme advance with the spark occurring closer to the F mark. Between S and F but closer to F. The S mark should be static timing with F being full advance indicated with a timing light. In that case, the spark occurred early enough in the compression stroke that it was inducing a backward shock wave that eventually shattered the nose casting of the starter motor. Be sure to double check the motor timing.

In the world of motorcycle maintenance there are Causes and there are Symptoms.

If we step out on faith and accept that the starter busted due to low battery voltage, then what caused the low battery voltage? Was it battery age? Too much battery "maintenance" charging? Poor alternator or rectifier output? Corrosion at the battery terminals? A poor connection at the gearbox ground bolt?

As dire as it sounds, I don't think you've reached the bottom of this pit yet. Probably some more digging to do, my friend.

I don't yet have another starter installed on the R90S. However I'm hoping the vendor will turn around the replacement fairly quickly. I'm also going to rebuild the original Bosch. Doesn't hurt to have a spare (whichever one goes in this time). Good riding weather these last couple of weeks. Bike looks sad sitting in the garage partly disassembled. Maybe time to do a spline lube as long as I"m waiting...

Ride safe.

-Gary

To answer Wobbly's questions:
> battery age: 2-3 years - AGM sourced from Batteries+, 30AH. Reading after charger indicated it completed: 11.79. The charger is a relatively new Battery Tender "smart" charger
> Actually the battery hadn't been on a charger for a few months. Sometimes if I shift things around in the garage I'm too lazy to scrounge an extension cord and I move the charger to another bike that's within reach. I think that's why the R90S was not on a charger just before the fateful ride.
> The alternator seems to work properly. It's a stock Bosch unit. I checked the brushes last season and they seem fine. The alternator light comes on with the ignition and goes out after the engine starts. It will flicker a bit when the bike is not quite warmed up and the off choke idle is a bit low. Bringing the revs up a few hundred RPM extinguishes the light.
> The rectifier is a board made by Thunderchild supplied by the PO (had never been installed). It's attached via solid mounts that came with it.
> Voltage regulator is an electronic 14.2 Volt non adjustable model purchased from one of the two well known electrics vendors
> New battery and starter cables installed last fall when the starter was replaced. Ground has been moved to the frame due to some damage around the gear box vent bolt. Didn't want to keep putting stress on the vented bolt.
I still have to take some readings from the charging system when the engine is running. I have a voltmeter I can install (the stock OEM meter floats and doesn't provide a great reading) to monitor the charging system while riding.