Ok. I'm not new to car repairs. I've done many a timing belt replacement. Replaced the timing chain on my 07 660. I got the engine at TDC, used the correct "I" mark, made sure the marks on the cam were lined up correctly. Turned the engine over a few times with a ratchet and socket, everything is good. Put the cam cover back on. Didn't install the cam chain tensioner until after installing the cam cover. Go to fire the thing up, cranks over a half revolution and "click", the engine locks up. Pulled the cam cover off, bent exhaust valve on drivers side. What happened?