I recently acquired a spare 6-cylinder motor but there was a spark plug sheared off in the head.
After multiple attempts I finally removed the threaded section of the plug but the seat was quite damaged....how to get into the recess to clean it up ?
Eventually I came up with the idea of a counterbore cutter.
A 14mm version had the correct diameter but the pilot was for a 14mm clearance hole (14.3mm) rather than the root diameter of the 14mm plug thread. I ordered a cutter from China via Ebay.
After speaking to a couple of people I thought the pilot would have to be ground down but I'm obviously out of touch...a customer said he'd turn it down with a ceramic tipped tool.
Pilot turned down but the sparkplug is recessed in the head so I needed to make up an extension. I found some 4140 hollow bar at work (used for guide rods on pneumatic cylinders). Drilled one end for an interference fit and cross-drilled for a tommy bar. I could have cut the hollow bar down but didn't bother.
Easy to manually clean the plug seats, I also used it on another head that had been left with the plugs removed to clear out dirt & corrosion in the plug recess.
Colin

