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Thanks, great advice as always. To be honest, I rather suspect that over tightening of the retaining bolt was the issue here-and that I am indeed the culprit. I also think the crank may have been a little worn where the spindle goes through it, thus meaning my rather over-zealous tightening forced the crank quite a bit further onto the spindle than intended. No injury this time as I was just starting away from some traffic lights, so I count it as a lucky lesson learned really (although the rider behind me near jumped out of his skin at the snapping noise). I shall try your oil/fit/ride/retighten method when fitting the replacement.
jeff80
@clubman
I 'm not the world's leading expert on the subject, but this is what I believe to be correct.
TA 314 is correct for a single chainring ( a double normally needs a 344). Newer 314 spindles are marked 111.
TA and Stronglight 49D are virtually identical, so no need to alter the BB spindle.
Steel right hand cranks (most makes) are made either for single or double rings - a crank for a double ring has a bigger gap between the crank and the ring to allow space for the changer, and the double type needs a longer spindle. I've got three old TA/Stronglight cotterless cranks here, and they all have the same gap - I don't know whether there was ever a crank of this type for single rings. For what it's worth I do know the 'Professional' adapter did come in both single and double form.
The 'double' type of crank will fit on a 314 spindle, but the inner ring will probably foul on the chainstay.
Since the TA and Stronglight versions are so similar I doubt whether there is any difference their strength and durability. If a crank breaks at the spindle end it could be metal fatigue or just bad luck, but I suspect the most likely cause is over tightening of the retaining bolt (not necessarily by the current owner!).
My own method for fitting these cotterless cranks is: oil the spindle, fit crank and tighten bolt but only using moderate force. Then ride a few miles (maybe five) and retighten the bolt. Check again after a few days. Opinion seems to vary about using oil, but I have found my method works, and it's worth bearing in mind that a shop does not want to have a bike back for checking and doesn't care if the crank won't come off the spindle in a couple of years time!