Hi Lindsay,

I have had a few of these in my time, and my findings are that the engine has been overheated, and it's always No:4 piston that melts. I imagine that the engine repairer would have put the old pistons back in, as they are £1000 with new rods because the piston was modified so you had to fit new rods as well.
we rebuilt one about 2 years ago that had overheated we removed the head which we sent for testing came back ok, we checked the pistons and bore for wear refitted the head 2 weeks later it was back with no compression No:4 piston melted

I just insist that i fit a mercedes exchange engine they come with everything on them including a new turbo just bolt it and go if they don't want that done i just refuse to do the job, the cost is not that much different time you have swapped all the parts across.

as i say this is not uncommon, and when you think about it, if it has lost all it's coolant which cylinder is going to get the hottest

HTH

Kev Gundry