Face seals, such as the PSS shaft seal by PYI, are also dripless shaft seals, requiring less maintenance than a packing box, and like lips seals, they get cooled by the engine's discharge water. Face seals rely on two machined faces pressed together at 90 degrees to the shaft. One face — the rotor — is split and bolted in place onto the shaft. The other half — the carbon face — is pressed against this split ring with pressure exerted by a sprung bellows. Because face seals do not seal against the shaft directly, they can provide a good seal if the shaft surface is less than perfect, such as an older shaft that has seen service but is still structurally in good shape, making them popular for retrofits. Face seals need to be regularly inspected for leaks, but in practice last for years.