Abstract: In foundry manufacture of metallic castings it is customary to employ inserts to form elongate ducts whether rectilinear curved or branched ducts and whether of regular cross-section or varying cross-section, including enlarged cavities. A core insert is disclosed which has a wire formed outer tubular casing and an internal wire structure, the core insert being removable in stages. Removal is accomplished by first removing a central longitudinal wire, then any intermediate layer is removed and finally by uncoiling the outer tubular casing, which has preferably previously been wound in such manner and of such diameter wire as to avoid, so far as possible, any exaggerated surface undulations.