Abstract: Apparatus for inserting full bore tools into an earth borehole, such as oil and gas wells, includes: a blowout preventer assembly superimposed on the wellhead of the borehole; a seal body having an internal concentric seal diameter, superimposed on the blowout preventer assembly; a tubular mandrel concentrically engaged with the seal diameter of said seal body, with a standpipe attached thereto; a tubular bell nipple connected to the seal body; and a pack-off assembly disposed between the internal diameter of the standpipe and a workstring inserted into the earth borehole.The blowout preventer assembly, seal body, and bell nipple are typically installed on the wellbore when well operations commence. During ordinary, unpressurized wellbore operations a seal diameter protector may be installed in the seal body.