I would try the typical foaming citrus engine degreaser first. Or, People have said that oven cleaner works. However, with the oven cleaner I would not leave it in there more then 2 hours. It will turn the aluminum black if left on too long.

If you are lucky one of the manifold studs will unscrew and yo will see that there is only about 4 full threads on the end that screws into the cylinder head. That means they cannot be tightend down a lot or they will strip.

When I removed the manifolds I never found the torque for the Nuts and it would be difficult to torqe some of them anyway.

I looked on the internet and did find a torque for generic copper manifold nuts but I don't remember the torque. Someone posted a torque in one of the threads but I cannot remember that either.

Take a straight edge across all of the precision surfaces especially on the exhaust manifold and see if they are straight across.

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84 300D, 82 Volvo 244Gl Diesel