The tap gradually cuts the 10mm slightly wider so that the thread is cleaned up, and you can fit the machined aluminium insert into the spark plug hole. If you have a cross threaded 10mm x 1mm spark plug in an Aluminium head, then this is the perfect little kit to rectify it.
KIT DE RÉPARATION de fil de bougie d'allumage pour fils de 10 mm x 1 mm dans têtes en aluminium EUR 13,68 - PicClick FR
KIT DE RÉPARATION de fil de bougie d'allumage pour fils de 10 mm x 1 mm dans têtes en aluminium EUR 13,68 - PicClick FR
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The tap gradually cuts the 10mm slightly wider so that the thread is cleaned up, and you can fit the machined aluminium insert into the spark plug
Spark Plug Thread Repair kit for 10mm x 1mm Threads in Alu Heads