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Old 02-10-2018, 04:16 PM
Innocent Bystander
Join Date: Oct 2016
Location: AZ
Posts: 132
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?

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Old 02-10-2018, 04:21 PM
Registered User
Join Date: Sep 2001
Location: central Texas
Posts: 17,277
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..
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Old 02-10-2018, 04:59 PM
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Join Date: Aug 2010
Location: Sacramento, CA
Posts: 2,543
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.
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans
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Old 02-10-2018, 06:56 PM
jake12tech's Avatar
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Join Date: Nov 2011
Location: Philadelphia, PA
Posts: 2,746
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.
1999 E300 220k
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Old 02-11-2018, 03:28 PM
t walgamuth's Avatar
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 34,780
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.

[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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