VW Scirocco Engine Repair

I towed the car to VW because the engine was misfiring. VW stripped the engine in order to make a diagnosis. VW discovered that only the piston was damaged, and subsequently the piston was changed. After replacing the piston there was a knocking sound coming from the engine and before they diagnosed there was no noise coming from the engine. VW told me that they could not help me any further unless I bought a new engine after spending R17200.00. What frustrate me even more is that VW informed me this morning that they are stripping my car engine again in order to find the reason why I need to buy a new engine and basically this does not guarantee that they will deliberately cause a damage on my engine to justify their reason why i need to buy a new engine.