Engine rebuild
 

On the subject of pistons; (240d)

If I get the bottom end machined I assume they will overbore the cylinders, then install larger pistons. What is the beginning size and what is the next size up? I dont find much distinction other than 90.9 but nothing in a step up (I was told by a mech that he used to go 0.020" up). Are they NLA? Where to source?

And what/why do they list auto or manual trans? Is there a difference? Does it matter?

You have been dealing with Normal engines..
You really really need to get a copy of the Paper
616 and 617na Factory Shop manual for you AND your machinist to STUDY.
MB thinks and does things differently.. that is why they are so good and last so long...
and I know you want yours to last a long time..

I have never seen over-sized pistons for these engines. They have removable cylinder liners, so you replace those and hone to match each piston. A few posts if you search (incl. mine). You are lucky since new non-turbo engine pistons are almost affordable ($100 ea vs $550 ea for turbo), though most people re-use used pistons.

What does "bottom end machined" mean? The only machining I know of is the crankshaft, in which case you turn it down to match a thicker bearing. That is only if the crank was ruined by no oil. Otherwise, you just replace the main bearings. In most engines, you can do that w/ engine in car, but not these since the upper oil pan must come off and that requires space.

You're not going to be doing any machining to the bottom end except doing liners and honing to the pistons like Bill said. Which you can do yourself at home. Unless you have seven hundred gazillion miles I doubt you need anything besides rings, if that. The bottom end wont go bad unless you run it out of oil. The top ends barely wear out as well, so first I'd do a leakdown test to see what's going on.

My machinist measures the pistons. There is a wear limit by the factory. If the pistons are not too worn we re-use them. I think I have only replaced them once and then It may have been with less worn used pistons.