Two-Piece Reversible Jaws; Jaws are composed of top and bottom pieces; American Standard Tongue and Groove Style; ISO3442 Highly durable; Steel inner mechanisms are quench hardened to increase toughness Plain back; Requires a fitted backing plate to mount to your lathe’s spindle; CME fully machined backing plates are available 250 mm Chuck Diameter; 80 mm Through-Hole Diameter; 6 mm-110 mm Clamping Range; 90 mm-250 mm External Clamping Range 206 mm Registration Recess Diameter; 5 mm Registration Recess Depth; Rear mounting bolt holes are positioned on a 226 mm diameter circle