Irolla, it's quite common to find the oil cooler by-passed on a D2 in the UK,
i've been lucky to find a few nos coolers in the past but they are like hens
teeth now.
shovel man.
On the J-series, the lines are rotted out that run up by the radiator, which I have been lucky to find a good replacement, plus the lines running along side the engine are rotted out usually, which I was lucky to have material to make them. But then the cooler sprung a leak, I gave up and bypassed mine back. I may find a good replacement cooler someday.
Most D2s and D4s have had the oil coolers bypassed because Caterpillar determined that they were not needed. Once they sprung a leak, they were disconnected and the machines suffered no ill effects. Late models don't even have the cooler.