All engine faults/repairs/maintenance Stripped sparkplug thread
Stripped sparkplug thread
A timecert or helicoil can be a cheap option as it is possible to put it in situ, however there is possiblity that fitting may result in swarf falling into cylinder. Can reduce chance of this by rotating engine so both inlet and exhaust valves are open then attatching compressed air to carburettor and hopefully blowing swarf out through exhaust valve. Best to take head off and do it proper. Syncro-Nutz put a timecert into my (engine) head. They are better than helicoil as they are solid, helicoils are like a spring and will probably let you down one day, maybe just before you go on holiday as is what happened to me [properly specified and fitted and inserted helicoils are normally fine in most applications - been used for many years - Ed]. Aircooled engine is more awkward to get to spark plug hole under the tinware as well.