I just used a tap of the correct thread, one time. It didn't seem to do much, other than clean slight carbon from the threads in the head. When I have had the injectors out, I always look down into the pre-chambers with the glow plugs on. I don't see any carbon, so not sure what the factory reamer is intended to do.

I am trying post a pic. In the Pic the area inside of the Red Rectangle is the area where Carbon builds up.

There should have been no Carbon in the threaded area of the Cylinder Head as the Glow Plug seals on the tapered shoulder (like a Spark Plug) on the Glow Plug Body right before the Tip/Heater Element sticks out from the Body.

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84 300D, 82 Volvo 244Gl Diesel