Comparing a Babbitt and a Roller Bearing spindle shows that they can be substituted for each other with a few simple modifications!  The Roller Bearing spindle has a large washer on the end between the nose shoulder and the bearing on the headstock end.  This washer comes loose when the roller bearing is removed.  The Babbit spindle has a dowel pin in smaller diameter area, as well as a small hole near the shoulder between the small and large diameter joint.  I am not sure what the purpose of this hole is.  The pin holds the spacer between the left hand Babbit bearing and the spindle threading gear.  Figure 1 shows a Babbitt spindle (bottom) and a Roller Bearing spindle (top).  The Roller Bearing washer is shown between the two spindles.  The washer has the following dimensions: OD = 2.250", ID =  1.503", Thickness = 0.190"  The two arrows barely visible on the Babbit spindle point to the remains of the dowel pin (I cut it off) and the small hole.

Figure 1

    The two spindles shown cover quite a span of time for the Atlas lathes.  The Babbit spindle is from an early 30's vintage lathe, and the Roller Bearing spindle is from a 70's vintage Atlas.

    Remove the pin and use the washer from the Roller Bearing spindle and the Babbitt spindle will fit perfectly in the roller bearing headstock.

    Drill a hole for a pin in the Roller Bearing spindle and remove the washer, and it will fit in the Babbitt headstock.

    The only other difference between the headstock setups is that the Babbitt headstock uses a ball-bearing thrust bearing between the small spindle backgear and the left hand Babbit bearing to take the load when cutting toward the headstock.  In the roller bearing setup a setscrew collar is used in this position to hold the spindle backgears and pulley in place.  The tapered roller bearings themselves take this load.

    So if you can find a discount on a headstock with a messed up spindle, you can use your spindle.  If you are replacing the present headstock with an entire new headstock, keep your original spindle as a spare!
 

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