hi jim
firstly measure outside diameter of thread from memory should be 10mm-then count 10 threads & measure length with vernier from memory this should also be 10mm so therefore if this corresponds the tap will be m10*1mm.but if however it was 12mm o.d. & ten threads measured 12.5mm the tap would be m12*1.25
hope this explains metric threads clearly
colin
if however thread is pulled out i use wurth timesert replacement inserts especially on vito engines