I wondered about the Pre-chamber Seal Rings and their different sizes when I read them in the Manual. You are supposed to measure the Pre-chamber protrusion and use the different sized seals to adjust that protrusion.
When you change the Pre-chamber Seal Rings while the head is still on the engine how do you guess what size of Pre-chamber Seal Rings to use? I am assuming that the old one gets crushed some to seal so measuring the old one may not help much.

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84 300D, 82 Volvo 244Gl Diesel