My dad does a lot of drum brake overhauls, mostly skimming drums and relining brakshoes, but recently he has also started repairing cracked drums. He has been riding BMW's for about 40 years now, and they have a problem with cracks in the cast iron lining of the drums. Moisture and dirt collect between the aluminium and steel, it oxidizes and the steel gets pushed inwards.
Here is a drum that has been turned on the lathe to cut out the lining, which came out in one piece.
Obviously this has a negative effect on heat transmission from the steel to the aluminium, causing the lining to overheat.
This drum will be prepaired to fit a new cast iron lining, here are two drums that are allready done.
The holes in the right drum are not standard it used to be in a race-bike.