Abstract: A film core is mounted on a mandrel by utilizing two telescoping sleeves. The inner sleeve is keyed to the mandrel to prevent its rotation about the mandrel is frusto-conical in shape and threaded at the smaller end. The outer sleeve, which fits between the core and the inner sleeve, has an inner tapered surface adapted to engage the frusto-conical inner sleeve. A ring nut engages the threaded inner sleeve and when tightened forces the outer sleeve, which is split, to expand and the inner sleeve, which also is split, to contract. In this manner the core is locked to the mandrel.