Predator's are pretty good, lasting engines. Most people i know dont swap engines, but just rebuild it.
A lot of the times, Piston slap (knocking): Piston slap occurs when the gap between piston and cylinder is too wide. Through wear and tear, the gap between piston and cylinder becomes wider. The noise associated with piston slap occurs in association with the perpendicular movement of the piston.
just a little lesson