There is disclosed an improved drilling head which provides for the formation of a continuous seal about a drilling string, where the drilling string consists of varying diameter drill string components. A stationary bowl configured to pass a drill string is provided with an annular grove about the top so as to allow for the attachment of a connecting clamp. An assembly carrier is provided which is removably disposable within the bowl and to which a lower stripper rubber may be attached. A bearing assembly adapted for placement within the assembly carrier is provided with an upper rotatable stripper rubber of smaller diameter that the lower stripper rubber so as to form a seal on smaller diameter drilling strings.