A customer presented a vehicle with an engine Knock, after removing engine and inspecting the bottom end of
the engine it was found that most of the bolts on the big end bearing caps where loose. On further inspection, it was noted that the engine had been dismantled for repair prior and appears the same bolts where used to reassembled the engine to sell the car.
Most manufactures these days will use one use item bolts only. What does this mean, It means that the bolts will stretch when tensioned from new, and once undone, cannot be re used as the bolt has already stretched to the point of no return and will never go back to the correct tension again.
So in this example it looks like this is what has happened, Over time the bolts work loose and eventually cause sever damage.
There is a mathematical formula for working this out. Known as- Torque to yield
When in DOUBT THROW OUT.