In an axial-flow preparative chromatography column that contains a piston, commonly termed an adaptor, that is lowered over the top of the resin, the ability to lower the piston in a controlled manner to minimize the chances of damage to the resin is achieved by use of an aperture either in the piston head itself or in a compound bolt that secures the upper column frit to the piston head. The aperture is closed with a removable plug during storage and use of the piston head and column. During packing of the bed, however, the plug is replaced by a graduated rod operating as a dipstick to allow the operator to determine the distance between the bottom face of the piston head and the upper surface of the column packing.