I have 3500 miles done since the build, the gearbox was feeling a bit awkward and I became paranoid about something breaking, the titanium valve retainers need replacing and I was getting paranoid about how the pistons were doing with all the power and my tuning.
So decided it was high time to tear the engine apart and inspect most stuff, while doing the work above including transmission upgrade (before something broke).
Unlucky that it broke but lucky it was caught before serious damage.
It seems the crank pulley spigot might have held it together, after it fractured.
It might just be plain ol' bad luck and was a weak unit, seeing as nobody has had the issue before that I know of.
But I'm not really happy with just replacing it and hoping for the best.
I do have some very vague recollection of the key being a tight fit, but that was in the crank side rather than the sprocket side iirc.
Have to check my detailed files to see if the sprocket itself was tight going onto the crank nose.
If it was tight, maybe thermal related expansion might have been a factor.
E.g. crank swoll more under heat than the sprocket, and cracked it.
The key appears a bit damaged too (surface damage) so this might be a clue.
FYI Kev I use Nev's tensioner.
Before I buy a replacement timing gear, I'll ask Bill Robinson if he'd make one when doing my transmission.
It would be the same or better than the Triumph one.
A photo of the gear and key after a few thousand miles on the stroker engine, as I was going fitting all the supercharger & Carpenter bits;