When installing the R4 on my 300SD 1984 (twice yesterday...don't ask), I found it best to leave the manifold bolt screwed in several turns but far from tight. Then adjust belt tension and final torque the 19mm. mounting bolts . This will bring the manifold into better alignment and you can then snug up the manifold 13mm.? bolt. Use Nylog to help keep the O rings in place. [The flat washer R4's are much harder to do than the O ring models ]

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)