Don't know about the exact size of manifold screws, but if you remove the manifold, the crossover tube must go out first. It's attached to each end (the bolt on the turbo end is located under the tube) : remove the bolts and lift it first at the manifold end then pull it out from the turbo end. The shop manual says to renew the gaskets at each end.

It may be unnecessary to remove the intake manifold to change GP but it certainly will make the job easier. I didn't found any description of the GP removal procedure on the CD but I've seen many posts on the subject on this forum.

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DR
1992 300D 2.5T 351 000 km