The rear seal for the crankshaft is one of the most difficult to replace. When they fail, your engine will be marking it's territory with oil puddles/drops anywhere you park. Apart from making a mess, it can also cause oil to come in contact with the clutch disc, which can deteriorate the material, and if the oil level isn't periodically checked major leaks can cause extensive internal engine or turbocharger damage. This seal includes the rear flange.
The perfect time to replace this seal is during clutch replacement. Even if the seal has yet to fail, it saves you from tear downs in the future.
