Thermostat housing mating surface damage/corrosion/pitting

[Ray2k]

2001 E38 728i (M52 TU engine), 102000 miles. The thermostat housing was leaking, so I embarked upon a complete cooling system overhaul as seems to be the thing to do here. All was going fine, but now that I've started to clean up the thermostat housing mating surface on the head I find that there's a small area of damage or corrosion as shown here:



It looks worse in the photo than it is, but you can definitely feel the depression when you run your finger over it. Hard to tell or describe how deep it is, but it's definitely nowhere near as deep as the new seal stands out on the new thermostat housing (a Behr one, looks identical to the original genuine one I've pulled out, which appears to be the original). The damaged part is not the area it was leaking from, although I see no obvious cracks in the old housing where it was leaking.

Does anyone have experience of small imperfections in a face like this either leaking or not leaking? Is using RTV sealant as well as the new rubber gasket a good or bad idea?

I don't think I have a choice but to try it and see what happens (other than take the head off and get it machined down, the thought of which makes me feel rather ill), but any other ideas would be welcome.

Also, is the rest of it clean enough? I've got all the rubber off with sealant remover and a sharp plastic scraper, but can't budge the other stuff that looks like limescale, and don't want to use anything abrasive on the aluminium. It feels perfectly smooth as far as my finger can tell. Again, not sure how perfect these things need to be to seal well enough.

Thanks,
Ray

[pbonsalb]

I’d probably use a gasket maker in addition to the seal to reduce the chance of having a leak. Something like Right Stuff or the equivalent.

[ross1]

Tough to tell from photos how deep the pitting.
If I were doing it and convinced the seal wouldn't seal I'd thoroughly clean the surface, wire brush the pitted area and use metal epoxy which will fill the pits and can be "machined" flat easily with a block and sandpaper. It's a low stress area, if carefully applied I'd expect such a repair to last.

[Ray2k]

Thanks both. I'm really tempted by all three options: nothing, plus gasket sealer, and metal filler/epoxy.

Had a slightly crazy idea yesterday and tried making an imprint with some blu-tac to see how deep it was, which kind of worked. Not very scientific but eyeballing the imprint against my feeler gauges I'd say the deepest point is just less than my smallest feeler gauge which is 0.40mm. The new gasket sticks out about 1mm from the plastic under no pressure, but the profile of the gasket where it sticks out is pointy and it can bend over. The corrosion is in the right axis to be a "gentle" depression along where the gasket sits. It's also mostly on what would be inside the gasket, so less likely to "blow out" from that point of view.

For adding gasket maker would you use it all the way round, or just on that area? It's the one-piece gasket (very roughly "B"/"8" shaped), so there's lots of perfectly good surface that shouldn't need sealant.

I think the filler option is most risky in terms of causing further damage, so I'll probably try the other options first.

Also any comments on how clean the rest is?

Thanks!

[Ray2k]

...also just found that you _can_ buy replacement gaskets on their own (the new 'stat came with one), which is making me feel slightly better about trying things and seeing what happens. If that fan shroud wasn't there with all its hoses attached I really wouldn't worry so much about just trying... .

[dworthy]

Having seen this happen a few times, all I have done is sand down the mating surface, then ensured both gaskets were in place, then pressure test it. The soft outer gasket will fill those small gouges typically, but of course ideal would be to smooth it all the way down.

Just be sure to use approved coolant, for that is what normally will keep those gouges from forming.