W210 piston damage?

[Edmund]

I got a 2001 W210 E240 automatic about 2 weeks ago. There was a tapping sound coming from the engine so I sent it in to have the valvelifters replaced. The mechanic replaced all 12 valvelifters and informed me that the engine is still making the tapping sound. He reckons this is due to the engine running at very high temperatures causing the top of the pistons to shrink. Now I have to decide between replacing all 6 pistons or buying a secondhand engine.

I know the alluminium tops of the pistons can be deformed due to very high temperatures. But how does this cause the tapping sound that sounds exactly like valvelifters...

Is this guy just trying to squeeze money out of me?