Like water through an oil cooler, these are the days of our lives...

Having once more gone through the process of assembling the oil cooler, fitting it, and then ending up with an oil sump full of water again, we took it apart and had the cores checked again. They did find one more very small leak in the core that was not repaired, and fixed that, but basically we could not believe that was responsible for all the water pissing straight into the sump.

So, for the fourth time, today we reassembled everything, crossed our fingers, and filled the cooling system again. We could actually hear the water piddling into the sump as we filled. (weeps....)

Now in a zombie-like state, I stripped the cooler back off again, and carefully prised everything off. We had figured by now there had to be a fault with the seal of the gasket that was allowing water to flow between the oil side and the water side. And sure enough....it's the wrong bloody gasket!!!

The Cummins V8-300 has oil transfer cases on both sides, and in the 300M (marine) version one case is replaced with an oil cooler. The gaskets for each side are identical...almost. It turns out the gasket we had from the set was for the transfer case side, not the oil cooler. The correct gasket is nowhere to be found.

Gasket in place after removing oil cooler. Notice in the right-hand end the elongated hole at top - which is water - and the smaller hole underneath it - which is oil. Notice the absence of gasket between them. Duh.

So, the big question is this: Am I a complete f***king moron? I will answer with a qualified "maybe". The cores leaked, that was the original source of the problem, but three more strip-and-reassembles to find the fault indicates that perhaps I'm not the most observant mechanic in the bay.

And the great news, according to the Cummins website the gasket is no longer available. Off to the gasket makers we go...