450 overhaul?

[kip Foss]

I have a friend that overhauled the engine in his 450 sl (I don't know the year) to the tune of $6,000. He, a mechanic with his own shop, says that although the car now runs very well it smokes. After he asked all the MB people he could find why the engine smoked he never really got a solid answer. Finally someone said that the reason they smoke after installing new rings is that the iron in the block hardens after 200k+ miles and even though one hones the block one will never get the rings to seat properly because of this hardness. He took the engine apart again thinking that maybe he had a cracked ring but found all to be OK. Back together with the engine and it still smokes. Being a mechanic he is ashamed to drive the car because of the smoke.

Has anyone heard of this '200k hardness' before. I have worked on cars for 45 years and have never heard of it but then I have only just started working on MBs. Any help would be appreciated.