D2 4J oil cooler

Most people bypass them as they feel it is an engineering overkill. There are numerous threads on here if you do a search.
To take it off, you need to remove the chaff guard in front of the radiator. The oil cooler on the J-series is in front of the radiator. At the top and bottom on the left side as you are facing it you'll see the manifold connections held on with 2 bolts. When you take these apart a copper crush washer should be inside of these. Most of the ones in the US leak in the piping leading up to the cooler as the dirt and dust and moisture would collect and rot out the pipes. A good way to tell is if it is leaking out from the chaff guard on the front its the cooler, if the oil is coming out of the back of the radiator, then its the pipes leaking.
I got a set of pipes to go up to the cooler as mine were rotted out, cleaned them, brazed up the pits and pin holes, made a new set of pipes to go from the engine to the radiator as these were rotted out also from moisture and sand. Cleaned up the oil cooler only to have a leak develop in the tubes of the cooler so I gave up and bypassed it!
Hope this is enough to get you started.
drujinin

about a month ago I finally changed the oil cooler in my 3J, and fabricated a hard nose and put a better radiator in the machine.

Got everything hooked up, took my time using the brass O rings that seal the connection between the lines and the cooler.

After running my D2 for several years with oil lines bypassed I was happy to get everything correct again.

I ran up the machine used it for about 4 hours work. Checked it no leak.

The very next time I ran the machine, oil leaking all over the place coming out of lines and cooler..

I went back to bypassing like everyone has ever told me:

"just by pass them!" They are a problem, and the last thing you want is it to start pumping oil out while your not noticing it, because it's way out in the front were it can be hard to see the oil pumping out the bottom side..

😕