Maybe not the most difficult, but surely one of the more frustrating repair jobs I've ever done.
The motor in my old 2.2L 911T was making a cyclical growling noise, and not knowing the motor's full history, I decided to do a full rebuild just to be safe. To save a few bucks, I replaced/rebuilt everything except the intermediate shaft that runs the cams...thought it looked OK. Got it all back together, fired it up, and that damn growling noise was still there!
Sure enough, the noise was coming from a worn drive gear on that shaft...the one thing I didn't replace. Had to drop the motor and take it completely apart again to make it right
