Abstract: A dual diameter centralizing sub for maintaining stand-off and/or centralizing a tubular member inside a larger diameter tubular member, for instance, in a wellbore. The centralizer is provided with bow springs that compress into grooves between radially outwardly-extending vanes that are spaced around the outer diameter of the sub when compressive force is applied to the bow springs. The vanes extend radially outwardly far enough that the effective diameter of the sub in the area of the vanes is greater than the diameter of the sub and/or the tubing to which it is mounted to provide stand-off even under conditions in which the bow springs are fully compressed while still maintaining fluid flow.