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What's going on here?

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Laverda Dave:
 Hi All.
A mutual friend contacted me last week about a problem he's had with a newly acquired 1976 Honda CB550.
He took it out for a spin and the kick-start lever suddenly flew down and stalled the engine. The engine would not then restart with the kick-start as it had seized solid!

He took the engine out of the frame and removed the sump to find the kick-start gear has seized solid to the kick-start shaft. He eventually removed the shaft by cutting the gear off.

He sent me the attached photo.

It appears as though the kick-start shaft has been spinning so fast it has welded itself to the gear. I know this engine has a ‘tang’ in the crankshaft casting to stop the kick-start shaft rotating. I don’t know if the tang has broken off as I haven’t seen the engine but the owner tells me there was nothing in the sump to indicate something has broken.

My concern though is looking at the burring damage on the journal of the kick-start shaft how did he get the shaft out from the engine without drifting it through and thereby damaging the crankcase bearing?

He’s bought another kick-start shaft as in the photo but that is only going to replace the damaged shaft, the potential cause of the problem and possible crankcase damage remains.

Does anyone know of what may have caused the kick-start to drop down in the first place considering the bike was being ridden at the time and if the ‘tang’ has broken off can it be welded back on to the crankcase and if it can will the crankcase need to be split to do so, I’m second guessing it will and whilst he’s in there it would also be a good idea to check and possibly replace the primary chain?

Thanks in advance for any advice.

Sesman:
This may assist in the diagnosis.

Sesman:
And the opposing case.

Sesman:
I’d say the kick start pawl retraction failed (spring and mechanism) causing the shaft to be permanently engaged.

Regarding the primary drive, I guess it depends on the current condition and intended use. I’ve attached a photo of a new vs 17000 mile example. I’d say the used one had a further 5-10 thousand miles in it at absolute best. And that mileage would be noisy and snatchy. You would also probably benefit from checking and potentially replacing the primary drive Cush rubbers which can go very hard. Though to be fair, mine were fine. I replaced them anyway.

Laverda Dave:
Thanks for the photos.
I can't see any tang there but looking at the photo of the shaft and the slug of metal left in the centre of the gearwheel after it had been cut off the shaft now makes me wonder how did the gearwheel travel that far down the shaft to seize on the shaft considering when you look at the replacement shaft and the shaft in your photo that should be the position of the ratchet gear, the straight cut gear should be hard up against the journal?

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