Where does the crank break?

[Ib-Denmark]

Hello
I had a discussion with a friend, where does a 650-crankshaft break? (Never seen one)
Is it always in the same position?
Is it only cranks with alternator?
Does nodular-iron cranks also break?
Yes I know it was only a few but am just curious.
Regards Ib

[Groily]

Only crank I ever had break was on a 500, and it broke on the inboard end of the left hand big end, adjacent the flywheel that side. It had been metal-sprayed, 35 years ago, from a worn -80, so perhaps not so surprising. The oil passage from the centre main to the big ends passes quite close to the surfaces of the big end journals at the inboard side, at an angle, so there's an inherent weakness there when the crank has been ground down . . . and down . . and down!
The design of the new batch of 650 cranks has the oil passage slightly more deeply buried as I recall, to reduce any risk there to a minimum.

[242017]

Hello Ib,

Were you not at Kerteminde IJR a couple of years ago..??....I had a broken Matchless G12csr crank holding down my awning on the van when I was there displaying my Matchless G85cs..I normally use the crank as a doorstop in my workshop.

The crank was from a bike I imported from the UK for a danish customer...he was not impressed...But then if twins are a problem stay single !!!

I have also personally broken one on a 1959 Matchless G 12 csr....when I lived in England

They brake on the left hand side journal next to the flywheel.

You are very welcome to come round and look at my "doorstop" the next time that you are over on "Devils island"

Best Wishes Geoff

[Duncan]

I have had two on the timing side, 650 alternator twin.

DSCF1109.JPG

This engine was still ticking over, albeit very rough and noisily, when I pulled over

[ajscomboman]

I have had two on the timing side, 650 alternator twin.

DSCF1109.JPG

This engine was still ticking over, albeit very rough and noisily, when I pulled over

And some say it's the quietest your motor had ever sounded !!!!

[nialdavies1976]

So this is great timing! I've just been told that what I thought was a main end bearing failure was in fact a crank snapping. I'm devastated as after 2 years of rebuilding this bike (a 1961 G12CSR) I'm now facing another summer of just looking at it.

Can anyone suggest were I might find a replacement, I tried AMOC spares AMC Classic Spares, Russells etc with no joy. Any suggestions are most welcome!

Thanks

Nial

**UPDATE** Les from the Hampshire section very kindly got in touch with me offering me his spare. Thanks Les!!

[nialdavies1976]

Another update, it turns out that the guy who reground the crank for me did not put a radius in the journals (there was a clear step) which is where it snapped leaving me with a rare doorstop.