I have a 1992 Toyota Tercel; it uses spark plugs NGK RN11YC4. The plug threads for cylinder #1 stripped out, and I repaired them with a standard rethread kit. After 60,000 miles, the repair threads blew out. My mechanic and a machine shop tell me there is no oversize repair kit, and I should replace the head.
The original plugs are 14mm. There appears to be space to tap out the threads to 18mm. Is there an 18mm equivalent to the NGK RN11YC4?