how big is that 1/2 inch??? you may want to find a washer with a similar shape to that piece and measure it... if its about piston pin size.... it may be time for a teardown or 408 go

 

There's no doubt in m ymind that's what it is. I just can't think of what could happen to make them come apart.

 

I figured you had snap rings, thats why I didnt say that. Whover put the spiral locks on the pistons if they didnt get the last part of the spiral lock seated in the groove on the wrist pin boss(which is easy to do) then with rpm/time it can come out.