In Reply to: Spark plug thread? posted by Farmer Dan on December 12, 2018 at 13:50:40:
This chart says use a 12.8mm or 1/2" drill bit for the fine pitch 14mm x 1.25mm tap. Fine pitch threads always use a different tap than their coarser thread counterpart. For example, a 1/2-13 thread size uses a 27/64 (.4219") tap drill and a 1/2-20 thread size uses a 29/64 (.4531") tap drill. The Ford spec'd spark plug, Champion H10, 14mm is the fine thread pitch (1.25mm)type. The coarse thread size would be 1.5mm, a .010" larger pitch. You need the correct 14mm x 1.25mm nut and sure, the H10 plug will screw into it just fine. I have both 14mm taps in 1.25 pitch and 1.5 pitch if you need one. Plenty of drills in both INCH and METRIC sizes as well.