Can anyone tell me the sizes of the four O-rings I'll need when replacing the expansion valve on my 1985 300CD?

That is listed somewhere in the archives...
but you can call Carlisle Auto Air in San Antonio...also in the archives..and find out... it may be the same part number as my 123 which used to use #100 for the R12 green ones...

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/showthread.php?t=156207&highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Replaced one a couple weeks ago. Ordered what I thought were the right o-rings but they weren't. Ordered a Santech aftermarket brand o-ring kit specific to my car and it had what I needed. The one for your 1985 300CD is MT2631 according the Autozone site. I know Santech is sold at other auto parts stores as well. They're usually between $9 to $17 for all the o-rings for the system, plus extra o-rings. They always come with more than you need.