Hey Connor,
The exhaust manifolds are obviously running a lot cooler than the heads if there is no oxidation on them. Since the engine is fresh water cooled, the risk of keeping those heads is minimal and I think I'd just put the new manifolds on it and press on, but keep a close eye on coolant level. If you see it start to drop, loosen the Hex cap to eliminate pressure in the system that would force coolant out thru any cracks. The gasket surface is at the extreme outer edge of the cylinder head casting, so I'm guessing the head is fairly thick in that area. If you can trim the gaskets to match the ports and eliminate any flow restrictions at the head/manifold joint, that would help. I'd trim the gaskets to match the ports on the head, and then see how they match up to the exhaust manifold. The old gasket might have been some sort of asbestos material that's no longer available, but carbon fiber would be an excellent material if you could find some, as it will probably withstand about 1900F! If the manifold is smaller than the trimmed gasket, then I'd break out a grinder and open up the manifold to match the gasket. Maybe use some of that muffler repair putty as a gasket sealant. It's basically a ceramic material and will take some fairly high temperature. Those siamesed exhaust ports are the hottest spots on a small block Chevy and are one reason the Fords tend to be a bit more durable in that area since they have individual exhaust ports. The big block Chevy and Chrysler Hemi also have individual exhaust ports to minimize hot spots. Interesting that they didn't even try to seal the area between the ports, but I think I've seen automotive exhaust manifolds with just one big opening for the center ports. That would explain why you've lost material from the rib between the ports, but that's no big deal. I'm guessing you didn't see any problems on the end ports.
Although I've heard that you generally can't justify the extra cost of FWC in a gas engine, this is one situation where it allows you to keep running a set of cylinder heads where it would be a lot riskier in a raw water cooled engine! Your valve seats are obviously in good shape with those compression numbers, so good thing you were observing the 7" limit on manifold vacuum! It's strange that the exit of the exhaust port is evidently running hotter than the exhaust valve itself or the valve seat area, although maybe Volvo uses better materials in those areas! Denny
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