| heads to get a range for machining the plugs. They are machined to the average of these dimensions with a necessary tolerance of ±0.001" for both the groove width and the radius to flat. The head's dimensions hardly vary by a measurable amount, but this does not take into account the possibility that the bore could be damaged. The cylinder head should be checked closely to make sure the plug's bore is smooth and doesn't have any dents. It doesn't hurt to run a piece of sandpaper over all the sharp edges to knock down any burrs prior to installation. My intent was to make the parts as tight fitting as possible so they would be more-or-less a permanent installation. Probably 99.9% will need to be smacked into place with a hammer - I recommend using a piece of wood as a drift to protect the half-moon from being dented by the hammer. As we can see from the original post, DO NOT rely upon the valve cover to push a half-moon into place. One last point - don't forget to use RTV between the half-moon and the head.
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