Penatrating Oil sprayed around the threads (PB Blaster is sold out here at Home Depot, Kroil is supposed to work even better but I have no experience with it) and letting it soak several day.

I think the 602 Injectors are treaded into the prechamber retaining ring. If they are badly stuck in place there is some chance of loosening a prechamber retaining ring and you will need a special tool to tighten the prechamber retaining rings and a torque wrench that can go a lot higher then the torque if the Injectors to re-tighten the prechamber retainer rings. Injector torque 70-80 Nm and prechamber retaining ring torque of 150-180 Nm. So there is an advantage to making sure only the Injector comes out.

Loosen the Injectors when the Engine is hot.

You need at least a 1/2 inch drive and a breaker bar 2-3 feet long.

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84 300D, 82 Volvo 244Gl Diesel