Simian's up to his arse in alligators atm, but I daresay he'll be\nposting more pix in due course.\n\nIt now looks certain that the ball race supporting the inlet cam, at the\ndrive end, failed, and this took the cam with it. It's possible that the\ncam snapped at the race, and took the ball race with it, but my money's\non the race going first. Either way, that caused the blow-up.\n\nThe question is: why?\n\nTo recap - we measured it, shimmed it, and torqued the cam cap down to\nexactly the right specified torque, tightening all the (20!) bolts in\nthe correct sequence. Put in the camchain tensioner unit, after first\npushing the plunger back into it, as per manual. Turned the engine over\na few times to check everything was OK, and that was it. 50 miles later,\nwhile it was being given max welly in fourth, it let go.\n\nThis worried the feck out of me because I was *certain* we'd done\neverything right, but coming so soon after a re-shim, it was suspicious.\nThe cam timing was right - or it wouldn't have run at all - and there's\nno way we over-torqued the cam cap. I even checked that the torque\nsettings in the Haynes BoL were correct (I've known them to be wrong\nbefore) and they were. And if it was *that* over-tightened I'd have\nexpected to have stripped a cam cap bolt, or broken the cam cap itself.\n\nSo..... possible over-tightened camchain, despite doing everything by\nthe book? Unlikely, and Simian tested the chain tension with his finger\nbefore we popped the cam cover back on, but still....\n\nWhen Simian removed the tensioner after the blow-up, it was nearly fully\nextended, but then as the cam snapped and the sprocket went flapping\naround, that would have put a load of extra slack in the chain, so you'd\nexpect that. And anyway, an over-tightened chain would probably take out\nthe exhaust cam first, since that's the one that gets most of the drive\nloading. And that's if it hadn't smashed the camchain tensioner system\nfirst.\n\nLast port of call was the tensioner blades. If the chain had been under\nserious over-tension for 50 miles, you'd expect to see signs of heavy\nwear on the composite surface. Apparently they look perfect. So it\nwasn't the tensioner.\n\nIt now looks as if a flaw in either cam or ball race was to blame. Maybe\na casting flaw in the cam, maybe a small flaw in the race, and as Simian\nsays, just shifting them to do the shims was enough to upset them.\nSounds ferkin' unlikely, but I remember when the main bearing race of my\nRD350 failed - sitting at traffic lights and all of a sudden the engine\nwent from a pleasant burble to a horrible grinding. The inner race just\nchose that moment to fail.\n\nAs someone (no names) here said in an email, Simian has been very calm\nand cool about all this. If I'd dropped my bike into a dealer for a\nre-shim and it went pop afterwards, I don't think I'd be happy. And yes,\nsome form of "arrangement" was offered if it turned out that (having\neliminated all the other likely causes) it turned out that the camchain\ntensioning was to blame.\n\nIf anyone's got any alternative explanations, please air them, because\n(and yeah, with a certain amount of relief), I've worked it down to the\nabove. Shit happened.\n\nAnyway, I've got the replacement lump in the back of the motor and am\nabout to trundle it round to Simian's gaff, because I'm going that way\nanyway (big sale at Thorne's, the bee equipment place in Windsor). It\nlooks like a bargain engine, too.