Had to pull a leaking oil pan off a Case 855E the other day - it had a hole worn in the bottom of it. As I pulled the last bolt & lowered the pan from under the machine, it had one of those rattling noises in it that you know is just not right. It came as a surprise because the engine was running perfectly with no worrying sounds at all. Well, what I found was a main bearing cap retaining bolt. Looks like the head had popped off & the remaining stud had fully unscrewed itself & fell into the pan. The bottom of the pan & bolt/head are very highly polished - I figure it has been in there for a couple of thousand hours. I fitted this engine as an exchange unit from Cummins about 6 yrs ago. Pulled the main & checked it out - couldn't see any problem with the cap or bearing. 2 x new bolts & a new pan were fitted - & it's back at work. Some people are just so lucky.....
Very lucky! I've seen main cap bolts come loose just enough to cause fretting on the block and cap. Good thing it was just one bolt.