Simian's up to his arse in alligators atm, but I daresay he'll be posting more pix in due course. It now looks certain that the ball race supporting the inlet cam, at the drive end, failed, and this took the cam with it. It's possible that the cam snapped at the race, and took the ball race with it, but my money's on the race going first. Either way, that caused the blow-up. The question is: why? To recap - we measured it, shimmed it, and torqued the cam cap down to exactly the right specified torque, tightening all the (20!) bolts in the correct sequence. Put in the camchain tensioner unit, after first pushing the plunger back into it, as per manual. Turned the engine over a few times to check everything was OK, and that was it. 50 miles later, while it was being given max welly in fourth, it let go. This worried the feck out of me because I was *certain* we'd done everything right, but coming so soon after a re-shim, it was suspicious. The cam timing was right - or it wouldn't have run at all - and there's no way we over-torqued the cam cap. I even checked that the torque settings in the Haynes BoL were correct (I've known them to be wrong before) and they were. And if it was *that* over-tightened I'd have expected to have stripped a cam cap bolt, or broken the cam cap itself. So..... possible over-tightened camchain, despite doing everything by the book? Unlikely, and Simian tested the chain tension with his finger before we popped the cam cover back on, but still.... When Simian removed the tensioner after the blow-up, it was nearly fully extended, but then as the cam snapped and the sprocket went flapping around, that would have put a load of extra slack in the chain, so you'd expect that. And anyway, an over-tightened chain would probably take out the exhaust cam first, since that's the one that gets most of the drive loading. And that's if it hadn't smashed the camchain tensioner system first. Last port of call was the tensioner blades. If the chain had been under serious over-tension for 50 miles, you'd expect to see signs of heavy wear on the composite surface. Apparently they look perfect. So it wasn't the tensioner. It now looks as if a flaw in either cam or ball race was to blame. Maybe a casting flaw in the cam, maybe a small flaw in the race, and as Simian says, just shifting them to do the shims was enough to upset them. Sounds ferkin' unlikely, but I remember when the main bearing race of my RD350 failed - sitting at traffic lights and all of a sudden the engine went from a pleasant burble to a horrible grinding. The inner race just chose that moment to fail. As someone (no names) here said in an email, Simian has been very calm and cool about all this. If I'd dropped my bike into a dealer for a re-shim and it went pop afterwards, I don't think I'd be happy. And yes, some form of "arrangement" was offered if it turned out that (having eliminated all the other likely causes) it turned out that the camchain tensioning was to blame. If anyone's got any alternative explanations, please air them, because (and yeah, with a certain amount of relief), I've worked it down to the above. Shit happened. Anyway, I've got the replacement lump in the back of the motor and am about to trundle it round to Simian's gaff, because I'm going that way anyway (big sale at Thorne's, the bee equipment place in Windsor). It looks like a bargain engine, too.
