How concerned should I be about the B2 in my 84 300D with 153K miles?
Looking through the maintenance records shows no transmission work except the 30K fluid and filter changes, so I'm sure it hasn't been upgraded.
How many miles do you have on your B2 Piston?

Don't fix what ain't broke. If your car recieved regular 30k mile tranny fluid changes, keep doing it. Don't worry about anything unless you are getting symptoms.

anybody else?

I'll second turbodiesel.... change the fluid and filter every 30k or earlier and don't worry about it.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k

Same here. Just maintain it.

If it's lasted this long, chances are the B2 bore will make it as long as the rest of the transmission.

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Never a dull moment at Berry Hill Farm.

Thank you for your replies. I was just curious if there came a point where I was literally begging for a breakdown. I would like to take the car on a cross country trip but was concerned about the inevitability of the B2 failure far from home. I think I will have the repair parts and instructions for the procedure in the trunk along with my spare fuel filter, belts, and hoses just in case. Thanks again.

IMHO, a B2 piston replacement is not a "do-it-on-the-side-of-the-road" proposition.

At the very least, accessing the B2 requires removal of the driveshaft from the transmission output shaft and removal of the rear crossmember so that the back of the transmission can be dropped far enough to get to the B2. You won't be doing that without a floor jack, stands and decent place to work.

Frankly, I'd subscribe to AAA and focus on the easy-to-fix things (belts and hoses) before leaving on a trip (a w123 with ACC has something like 15 hoses) and only carry fuel filters, some bailing wire (no joke!), duct tape.

That way, if your tranny craps out, have AAA tow you to the nearest town where you can rent a U-haul and an auto transport trailer then use your MasterCard and pull it home yourself.

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Never a dull moment at Berry Hill Farm.

I see your point and agree. Yes, the B2 is not a roadside repair. Carrying the parts and instructions would help if I ended up stranded in an area where the only mechanic knew nothing of Mercedes. Probably better yet to just tow it home as you said. Thanks.