http://forums.bimmerforums.com/forum/showthread.php?t=1796048
See post 12. The idea is that as the crank journal and the worn bearing pulls the rod down, a gap will form the bottom rod bearing and the crank. Visualize the crank pulling the rod/piston down. In a good bearing there won’t be much gap. I don’t recall the spec off of the top of my head, 18 tenthou maybe? A bad bearing could have 2-3x that.
To do the test you turn the crank until the suspect piston is traveling down. Then you insert a rod thru the spark plug port, allow it to touch the face of the piston and then bonk the rod firmly with your hand while carefully listening to the sound made. If the rod bearing is good the rod shouldn’t be able to travel hardly at all on the crank journal and if it’s bad there should be an audible thunk as the rod shifts. Then you do it a 2nd time to see if the character of the sound changes. If it does change, then movement occured during the first bonk on the rod and the bearing is likely bad.