There are usually only ever two parts that rarely break on an Alaskan Mill MK III. The U Bolt due to over tightening and the cast end bracket. This can break due to the mill not being square, over vibration due to poor chain sharpening and tension in the mill. Keep one to hand if milling a long way from civilisation!
However Granberg have now developed a brand new end bracket made from thick heavy duty aluminium channel. The clamp that mounts to the upright post has also been upgraded. You will unlikely ever break these!