The water distribution tube is basically a tube that runs front to back inside the block behind the waterpump to make sure that cool water gets all the way to the back of the motor. It's roughly D shaped, as long as the engine, and has holes in it where the cylinders are. if it corrodes away, or you forget to put it in, the rear cylinders will overheat because the water won't circuate all the way to the back of the block.
