Thx a lot for the links and the advices!
The oil cooler is 100% detached from the Engine
and the tube stack has been completly out. Just by Visual inspection
it looked fine.
I did put it all together again yesterday for future tests.
As I see it there are three possible causes:
1. The tube stack is broken - but it looks ok
2. The O rings are worn out - but they look ok
3. The only bad thing I've found is that the tube stack "edge" prior to the O ring is a Little damaged/corroded
I will tear it apart again tomorrow to measure the tube stack prior to asking it they are possible to obtain. I will share a photo
of the damaged part.
Taking friday of to conduct some sort of test using air while holding it in a bucket of water.
As a desperate act I actually bought a new Bowman oil cooler dimmensioned for 4.236 when I bought some other stuff.
But getting it in Place would require a massive rebuild
I will give eBay another try, so far I haven't found the magic Word that gives the right result. I find oil coolers for the 400 series but they are different.
Back tomorrow with some pictures