I did look at the threads as best I could manage with an inspection mirror prior to my first post. I could see not obvious damage, but it is a small hole and the threads start a ways inside. I cleaned the internal threads last night as well as I could with a toothbrush, as Pete suggested. I can just start the plug in by hand but I need a wrench to turn it from 1/2 turn onward. I am still unsure that I have proper thread engagement.

I picked up a 12-1.25 tap on the way to work this morning. I will try chasing the threads tonight. I initially hesitated to try that because I could see no good way to remove any chips I created and the idea of having an unknown number of cast iron chips bouncing around in my combustion chamber was not appealing. The idea of using grease in the flutes to hold the debris is clever. I will do that. I will also try using a drill bit to remove as much carbon as I can.

The old plug is also hard to get started, as is another that I have on hand. I assume I just have a problem with carbon buildup. The old plug squeaked like a dry hinge with every turn of the wrench while I removed it.

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1985 300SD, 203,000 miles, dark blue

Lil Sarah says: "Poor old Grandma Blue Car...She's sad."