Shouldn't need to pull the head. Invest time into making certain that the tap is started on the correct angle. A long dowel fashioned into an adjacent spark plug hole would be a good reference. The longer the turning tool (extension) on the tap the better you'll be able to sense the angle. The side-to-side angle should be easy to see, it's the up-down that isn't so easy to get "spot on."

Good luck!!!

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