[85RedCarrera]

You are screwing steel spark plugs into aluminum threads…it is the aluminum that is wearing/moving every time you remove and replace. After some time in the head, if you don’t use any product on the threads, the plugs will cause dissimilar metal corrosion of the aluminum threads and they will be difficult to remove, which means you are wearing/damaging the aluminum threads even more when you apply more effort to remove them.

Aircraft heads and motorcycle heads are the same - aluminum threads. Always use a lubricant with spark plugs going into aluminum threads. The old way was using a drop of oil on the spark plug threads. Anti-seize, if applied should not be applied to the last two threads, to avoid contaminating the electrode (which is a real thing). If the manual says lubricate the threads and gives a torque value, and you are not lubricating threads, then you are not torquing the plugs to the specified torque value- you are simply torquing until you reach a thread friction value.

These old heads are getting rarer every day- do the next guy/gal a favor and lubricate your spark plug threads. Every time. Clean the spark plug threads after removal, and lubricate the threads before re-installing.

At the very least, put a drop of oil on them before re-installing. There is nothing to be gained by installing plugs dry, except that they might be less prone to loosening- but if you lubed clean threads and used a torque wrench, wouldn’t loosen anyway. My .02 from a lifetime of working on air cooled motorcycles and aircraft engines.

FYI- Molykote is the good stuff….