My 1956 D2 (17478) does have a oil cooler but it, like the earlier ones, is a separate part of the radiator.

OM's comment on the pony engine struck a cord. I do have a significant amount of oil on one cylinder (none on the other) and had plans to pull the head to check it out later. That might be something to do and possible kill two birds with one Cat repair.

I really appreciate everyone's great help.

Tom

OM.
I believe that line and fitting you list is inside the valve cover and if it leaks, it will leak oil that was supposed to lube the rocker arms back into the sump. No possible connection to the coolant passages.

the most logical places are still the head gasket/ferrule area of the main engine, the pony head gasket and the PC chambers.

I didn't look far enough in the book to see them first time, but larger passages use ferrules and seals on the D311 just like the smaller pressure oil passage. If there's a leak, this normally results in a seepy/messy head gasket, but no leaks between coolant and oil unlike a standard gasket without the ferrules. I know CAT recommended coating the gasket with permatex on the D318 to stop the seeps and someone may have done it as well on the D311.

CC at the dealer we always used #2 permatex on all the head gaskets back in that era of tractor & Ind. engines.The D311 & D315 & esp. the D318 with the longer head had a seeping leak problem on lots of them along the outside edge of the heads so the #2 prevented it in the future from leaking.

CC,
Yes, that is true. Even if the ferrule connection at the head/block interface for the oil line was leaking it is not likely to get past other ferrules and seals to get to the cooling system.