1-1/4″ R8 Shell End Mill Holder ~ Hardened and ground. ~ For Bridgeport type mills. ~ Diameter range of standard end mills is 3 to 3-1/2″. ~ Drawbar end thread is 7/16″-20. 1-1/4″ R8 Shell End Mill Holder Hardened and ground. For Bridgeport type mills. Diameter range of standard end mills is 3 to 3-1/2″. Drawbar end thread is 7/16″-20.