Hi, what's the minimum thickness of drum wall before change is necessary?

I think # is cast onto the drum. It's the maximum usable width. After this point the brakes shoes don't match the circle profile.

Volvo brake drums don't have the maximum inner diameter cast into them like a modern brake drum has to by law.

I have never seen a printed maximum diameter listed anywhere. So we decided to reference other 9" drums and most them have a .060" maximum so that is what we adopted.

But that was before the brake drum price went to $300.00 apiece. I have seen some brake drums that are allowed to go .090" over nominal. And I have removed more than 1 brake drum that were so oversized, I had to adjust my drum micrometer to properly measure them and that means the brake drums were over .125" over maximum and there were no obvious problems with the wheel cylinders.

So we now will go .090 over 9" before we insist on replacing the brake drums.