Sometime in the past, a previous owner removed the oil cooler lines from the filter base to the radiator on my D2 (4U1826) and replaced with a hose that routes the oil from the output in the filter base back into the input.  By the looks of the condition of the ports into the radiator, looks like this has been that way for a while.  I'd have to clean the oil passages in the radiator and make sure there aren't any problems that caused them to do away with the lines before I put this back, and that is not in the plans for the near future.  Should I be ok running this as is?  Wouldn't be running long periods of time or under heavy loads, maybe one day I'll plow with it.

many guys have run there D2's without the oil cooler me included--the oil does stay cool enough without the oil cooler

As long as the plunger is removed in the filter housing it will be fine. Later engines done away with the oil cooler and went with a full width radiator core.

usual reason is the cooler started leaking. we fitted a aftermarket trans cooler for a land cruiser to our d4 works perfectly. just had to make a couple of brackets to mount it in front of the rad and plumb it in. unless your in really hot enviroment and working it hard not really needed.

Thanks everyone. That's what I was hoping to hear.

I have two D2 engines. The 5J runs in a small bulldozer and I have never been able to run it hard enough to get the oil anything near "hot". It runs the stock il cooler and still doesn't leak.

The other is a 5U engine converted to a generator engine. Once started and warmed up, it puts out about 18 kW steady--no letup. I have fashioned a home-made oil cooler and it helps, but the oil gets pretty hot. I can still put a hand on the filter housing and not get burned, but you wouldn't want to hold it there.

I think the question involves the service to which the engine is applied. On a generator, a pump, marine power...get an oil cooler. On a tractor constantly moving toward cool air and pulling variable loads, maybe not. But, I live in the Northern Oregon Cascade Mountain area. I have never run either in 100+F heat!