With the brake drum removed, tap it with a spanner and note if there is a clear ring; if not, the drum may be cracked. Deep grooves or scored internal surfaces are caused by rivets due to non-renewal of worn linings. If the brake drum has worn oval in shape, the amount of out-of-round can only be accurately checked using an internal type vernier or dial gauge. The maximum out-of-round and the amount by which the internal diameter of the drum can be increased by re-finishing are specified in the relevant guidelines.