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If the insert is missing, the hole in the drum is M12x1.5

Standard M12 i.e. coarse, is 1.75

OK - good to know. Looking at the parts manual I can see that I used M10 bolts. I didn't notice any inserts.

I don't know why they chose to tap M12 holes and then fit M10 inserts.

Cast iron is fairly soft/brittle and wouldn't last long. 12mm just happens to be what you need in order to fit an M10 insert (which is made from hard spring steel)