On engines with mechanical valve adjustment, excessive clearance causes noise from the valve train.

The high pressure combustion gases are released into the barrel at a rate which generates greater velocities than a mechanical valve could allow.

The way the blood flows through it should also reduce the risk of blood clots compared with mechanical valves.

Although mechanical valves last for decades, they require the lifelong use of blood thinners and, of course, carry the physical toll exacted by invasive surgery.

Many use mechanical valves made from smooth carbon.