The 5/32" chuck holds drills from 5/32" (4mm) down to #80 (.0135" or .343mm). It is designated "0JT" which stands for "Zero Jacobs Taper". This chuck comes with a #0 Jacobs tapered hole in the back. Into that Sherline has pressed a special arbor that provides either a #1 Morse taper to fit into the Sherline headstock or a #0 Morse taper to fit the tailstock.