When I did the ones in my 86, I used Wellman. Some of the old ones were really tight and I had to thread them in and out some to break up carbon. While unthreading them, I kept soaking with deep creep, which helped some. On a few I had to use pliers to twist and pull. If you have to do that, be sure to pull in a straight line with the plug so it doesn't break off. Most of all, be gentle and patient.
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