4″ 1″~8 Threaded 3-Jaw Backplate ~ Fits most plain back chucks. ~ Must be machined for concentricity. ~ Made of high grade casting steel. ~ Assembly instructions included. ~ Bolt circle is 3.3″. ~ Number of bolt holes is 3. Fits most plain back chucks. Must be machined for concentricity. Made of high grade casting steel. Assembly instructions included.