Big Ben cracked after two months use because its hammer was too heavy, and remained silent for four years. Rather than lug the cracked bell back down the tower and recast it again, the Astronomer Royal eventually came up with a better (cheaper) plan. The bell was rotated (through a quarter turn), a small square was cut out of the soundbow (to prevent the crack from spreading further) and a smaller hammer was installed (weighing ‘only’ 200kg). The crack gives Big Ben its distinctive (but less-than-perfect) tone.

via Diamond Geezer