https://www.tal-ko.com/PDF/FormulaTKM2-StrokeRegulations2017.pdf C3.4.19 It is permitted to use Helicoil type and Timesert thread replacements to repair all stripped threads on engine fixings on the crankcase and cylinder barrel. On the spark plug thread only a Helicoil type thread repair is permitted. Any other type of repair or insert is prohibited. Such repairs must not be used to derive any benefit other than rectification of damage. In the case of the spark plug thread, no portion of the Helicoil may protrude outside of the normal thread area. The coil must be inserted to the full length of the original thread and only one continuous coil to be used per repaired thread. In all cases the size of the repaired threads must remain as standard. On the carburettor, it is permitted to repair the non-metric threads with M3 or M4 threads providing they do not perform any other function. HTH
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