Bad news and good news from the machine shop. The bad news is that the other pistons have worn out ring lands, so the total failure on number five piston probably wasn't just a fluke. They're also saying the cylinder walls are beyond service limits, so they need to be redone. Given that, I'm leaning towards a set of oversize pistons and overboring it. I didn't want to bite that bullet, but at this point it seems like the best overall solution. So, does the rotating assembly need to be balanced when replacing the pistons? I've gathered from reading around here that Mercedes matched pistons to the rotating assembly, and so switching pistons affects the balance. Is there a specific process to balancing a 617, or can any experienced shop do it?

As for the good news, they said the valves and seats had plenty of material left, so they went ahead and ground them. They are recommending that I replace the valve guides on the exhaust only, the guy I talked to said he thought the intakes were tight enough, but that all his book told him was to use a Mercedes go/no-go tool for checking it, which he obviously doesn't have. I didn't have any way to measure anything when I disassembled the head, but I did notice an appreciable amount of play in all the valves, some worse than others. Can anyone point me to the proper measurement process and/or give some general advice? Would I want original sizes on the guides, or oversize? All I really have to go on is the Haynes manual, which is hardly comprehensive, and I don't know how accurate their instructions and measurements are.

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Once and future king:
'64 Ford Fairlane w/approx 238,000 - looks rough, but amazingly reliable if you know how to look after it; I will soon begin work to totally restore and modernize it.
Family vehicles that I lay some claim to:
'78 300D w/approx 350,000 original, '62 Ford F100 4x4, '90 Ford E150 w/171,000 original