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Bushings are small pieces of metal used to align and size pen blank tubes on a mandrel.  Additionally, they can be home made of corian or acrylic.  The quality of a bushing depends upon the accuracy of the machining used to create the bushing.  Because the fit of the pen tube to the fittings is critical to a quality pen, always use a caliper to finish the pen blanks to the exact size of the fittings.

These are standard pen mandrel  bushings.  They mount on a standard pen mandrel.  On the left is a standard 7 mm bushing intended for Slimline and similar pens.  In the center is a set of bushings intended for a pen that only has  a single tube.  On the right is a set of three  bushings for a standard pen having two barrels.  Some bushing sets have three or four pieces depending upon how they are made and purchased as a set.

This is a turning between centers bushing.  It differs from the previous bushings in that there is no hole in the center.  Instead there is a 60° recess in the bushing.  Only one barrel at a time may be turned using these between centers bushings. 

This photo shows the 60° recess in the end of the bushing

A list of bushing sizes is located here.