A multiple part mandrel on which a hollow body may be formed includes multiple sectors. Each sector has a skin on an outer face and a pair of opposed side returns at longitudinal sides of the sector. The mandrel formed by the sectors is symmetrical about a central axis of the mandrel. When the sectors are assembled as the mandrel, the side returns of adjacent sectors face each other. Each of the side returns includes a recess in an outside surface of the side return, spaced from the skin of the respective sector, and an inflatable O-ring disposed in the recess. The recesses of adjacent side returns in the mandrel are offset with respect to each other in an axial direction of the mandrel. Inflation of the inflatable O-rings urges adjacent sectors together at the skins of the sectors to seal the adjacent sectors at the skins.