Whilst trying to get a recalcitrant bike to start today, I ran into starter motor trouble. I'd appreciate any suggestions as to the cause: The motor was successfully turning over, but not firing. I pulled a plug and lay it on the cylinder head, and cranked the motor with the starter - weak spark, so I cleaned it a bit and tried again. This time, the starter motor was sluggish - classic flat battery symptoms but the headlight wasn't dimming, even when the starter button was depressed. I tried a few more times and the starter would barely turn over, or not turn at all. The battery was showing 12.8V - so it isn't a flat battery. After thinking for a bit more, I removed the starter motor and the shaft felt very "gritty" when I turned it - like a worn bearing does. I opened the starter motor to find that the carbon brushes had broken off, as had one of the copper tongues on the commutator. Another of the tongues was loose. The starter was full of carbon grit from the brushes. I'm not a big believer in coincidences, so it seems odd that the starter would suffer a catastrophic collapse just when I was working on the bike. However, since the brushes normally slide over the commutator without interfering with it, I can't think of anything I might have done that could have caused the sort of damage I saw. I've seen worn commutators - you can usually refinish them with some wet&dry or by skimming them in a lathe - but I've never heard of one breaking. There was no sign of burning or arcing on any components, and the broken commutator tongue was clearly mechanical damage - the copper on the fracture line was bright and clean, while the rest had the darker color of oxidised copper. The tongue itself was twisted and lying in the motor housing, clearly it has been broken by something catching on it. For the curious, the motor is a Denso and there's an exploded view of a very similar model at http://www.dansmc.com/denso_starter1.JPG Can anyone offer an explanation (preferably based on expertise rather than tea-leaf reading) of why a starter might collapse like this?