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What Stan said.
I think my heads are SCAT, and the size plugs they take are more commonly used on motorcycles. They're easily accessed and removed with a standard Craftsman 18 mm, 3/8" drive plug wrench, and have 12mm threads.
Carbs are Weber IDF40 on EMPI manifolds.
Another thought is that your plug wrench may be contacting the holes in the tins and not the heads themselves.
FYI, I replaced plugs after about five years. They were a little worn, but not too bad. and I hadn't noticed any engine missing. After another two years, I checked the new set, and they did need to be re-gapped a bit. It's now about eight years on this engine, and I should probably replace the plug wires on general principle.
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