First ride yesterday on a 1972 Commando purchased last year after a lot of work on various. I didn't strip the engine as it is running sweetly, with no unusual noises. Only rode about 3 miles so I could check everything was OK, but when I arrived back there was a clear scraping noise from the primary chaincase cover. Stripped it this morning and found that the alternator rotor and locking nut have been fouling the chaincase cover - see photos attached. This noise was not there when I set off so it seems to suggest massive crankshaft end float.
Will take the alternator stator off and see if there is any obvious play in the crankshaft. Has anyone come across this before and can shed some light on what is going on?
Perhaps the previous owner put in roller main bearings instead of the proper blended type?
Definitely worth removing the stator. My first check would be that the rotor (aka 'exploding' type) is not breaking away from its centre. This is a known issue. If it is that then it would be a 'good catch' as the results of the rotor breaking-up can be spectacular.
The 'chisel bodge' marks on the nut will not be helping although it does look as if the nut is rubbed opposite these marks as well. I'd check that the rotor is sitting correctly on the woodruff key and that the washer is of the correct type (not too thick, see Andover Norton 06.7894) and the nut is fully tightened.
'Superblend' bearings only existed in reality for a brief period of time (see here for details https://www.nortonownersclub.org/sites/default/files/Main-Bearings-v1.1.compressed.pdf and Roadholder 352 and 366 articles) and were to deal with the effects of crank flex not end float.
I would be surprised if there is enough crankshaft end float to cause this problem but 'stand to be corrected'.
First remove the stator. This will allow you to grab hold of and push/pull on the rotor. This will inform you if there is endfloat on the crankshaft or the rotor is loose.
Next pull off the rotor and check for spare washers on the crankshaft between the rotor and engine sprocket. Oil and/or magnetism would hold an unwanted washer to the back of rotor.
Check the run of the triplex drive chain. This will tell you if the engine sprocket is out of line and possibly about to wander up and down the crankshaft.
Is that nut actually tight on the shaft? Why is the end of the shaft so far inside the face of the nut? The Dommie has a special spring washer behind the nut. Without it, the rotor can vibrate and rock loose. Mine once destroyed the stator because the washer was missing.
Thanks to everyone for all the helpful comments. I took the stator off yesterday afternoon and pushed and pulled on the rotor to look for end float, but there doesn't appear to be anything abnormal. The engine sprocket is also in line with the clutch sprocket, so the primary chain is also perfectly in line.
I am interested in Andrews's comment about the rotor, as I am aware of the exploding version on the 1972 model. David's thought about why the end of the shaft is so far inside the nut is also interesting, so I think the next step is to take the rotor off and carry out the other checks everyone recommended. Interestingly the front of the rotor is in line with the front of the stator, so that doesn't suggest that the rotor is too far forward.
One final thought, the rubber chaincase seal was compressed completely flat when I removed the outer case, so could it be that it had been overtightened pushing it too close to the stator?
No, it takes a set and goes flat, the fit of the outer cover is set by its boss for the fitting nut and the corresponding boss on the inner cover with its bolt. If someone has been messing with these bosses then the cover outer to inner distance can change.