Legislation for environmental protection requires a total recirculation of all blow-by gases of an internal combustion engine into the combustion process.These nonburned gases which are caused by the inevitable leakage between pistons respective piston rings and cylinder penetrate into the crankcase. Efforts have to be made to suck off these gases in order to redeliver them into induction tract of the engine. This process creates a partial vacuum which might cause a sucking of air under the lip of crankshaft seals into the crankcase (Fig. 1).An uncontrolled feeding of air might be detrimental to the quality of combustion process and of emission control systems. So design- and test-engineers of the automotive industry need reliable data in respect to the amount of air sucked under the lip of radial shaft seals.