Oil Cooler Flange Gasket Leak Mystery

I have been dealing with a tiny leak in my oil cooler for the last couple weeks, where I find just a couple drips on the garage floor. Suspecting that the oil cooler needed new o-rings and a new block flange, I went ahead and tore into it this weekend.

I replaced all of the o-rings, then pulled the oil cooler block plate, and replaced the block gasket with a brand new FelPro gasket. The old one looked ok, but replaced it just in case. In the end, I reinstalled everything, filled with coolant, and had a large leak from the upper area of the cooler. Figuring that I messed up the upper o-ring, I pulled it apart, and tried again. Same issue.

Long story short, I left the oil cooler off, blocked the upper port, and the radiator hose, and refilled the coolant, to find that it was actually coming from behind the bolt right under the oil cooler. I had cleaned both surfaces, pulled and reinstalled three times, and in the end, I swapped the old (red one in the pic) one back in, and no leaks. I am guessing that the FelPro gasket (blue one) was defective, but I have never had an issue where the raised rubber part of the gasket failed and allowed coolant out a bolt hole.

Curious if anyone else has run into anything like this. Took my whole afternoon to fix something that should have taken an hour. Trying to decide if I should run to Ford and get a factory gasket, or just consider myself lucky and button it all up.

Did the kit you picked up come with two gaskets? Theres a gasket for the aluminum block and one for the iron block. I don't remember what's different, but make sure you have the right gasket.

Interesting. I just got the blue gasket from Napa. They never said anything about alum vs cast iron. I did give them my year, but my block was swapped with an aluminum Aluminator shortblock.

Any idea what year came with an aluminum block factory?

None

The aluminator block is what was used in the 05 up mustangs I thought

I recall a couple threads over the years

Blue gasket leaking vs orange.

Strange but guess it’s like some oil filters tick on this part.

slo984now said:

Did the kit you picked up come with two gaskets? Theres a gasket for the aluminum block and one for the iron block. I don't remember what's different, but make sure you have the right gasket.

Click to expand...

This. The gasket for all aluminum blocks is different then the iron pieces.

You need a gasket for an 05+ car since you have an aluminator.

Ok I’ll see if I can find the one for the 05 and later engines. Thanks

vdubn said:

Ok I’ll see if I can find the one for the 05 and later engines. Thanks

Click to expand...

Ok, I just figured it out. Both the red (what was on my car) and blue (bought at Napa for a 2001 Cobra) Felpro gaskets were the early style. The red one almost sealed the surface and appears to have been leaking since the engine was replaced.

I purchased a 2005 and later gasket today (also blue) and it became obvious that the gaskets I had were too short in the coolant area, which is why they would leak once the block filled up. I took pics showing the gaskets on the block, which is where it became obvious. When I put the gaskets in the flange, it looked totally fine, which is why I was confused. The old gasket wasn’t full squeezed in between the flange and the block at top, you can see the outline where the block cut into the old gasket.

I haven’t put it together yet, but I’m confident it will fix the issue.

Scroll down to the oil cooler section, the gasket differences are explained there.

Cooling

--Joe