I've been racking my brain trying to work out exactly how this happened and the only thing I can think of is that I screwed up! I pulled away from a stop sign and just after I changed into second and opened the throttle there was a noise and the engine stopped. Since I was just around the corner from work and the bike was still rolling I coasted down the road, round a round about and into the yard where the problem became somewhat apparent.
That was just before Christmas. So during the two week holiday I took the gearbox and primary apart, found some interesting things whilst I was in there. The clutch basket fell off when I removed the primary cover, the centre nut had come undone and the key had sheered. I had no symptoms of this before hand so I can only assume it came loose when the gearbox locked up? Also found that the clutch centre had split open and was being held together by the steel plates.
Looking at the chain I could find no evidence of rubbing and it had come apart at the master link. I found the master link in road. This leads me to believe that I somehow managed to not fit the spring clip properly. After 30 years of fixing motor cars this leaves me somewhat disappointed in myself!
Anyhow, nothing for it but to fix it. Once I got the chain free I was left with quite a hole.
I had a strip of 1/8 inch aluminium left over from making the front exhaust strap used in 66. After removing the jagged edge left by the break, I cut and shaped a piece of plate and epoxied it in place.
Then mixed a little aluminium weld through primer with some matte black to get a half decent colour match and touched it in. After using the bike mostly every day since the start of January it's starting to age nice enough.
I may yet muck around with the diagonal join but for now its good enough.