A tire building machine otthe expandable bead lock type employs an elastomeric specially shaped radially expanding bead lock (43) including a shaping and turning bladder (48) which includes an air passage system therethrough. The elastomeric bead lock is confined for radial expansion in an annular outwardly opening chamber (114) in the form of an isos- celestrapezoid in radial section with the elastomeric bead lock being of similar configuration and expanded by direct application of air pressure within the chamber. The configuration of the annular chamber and the bead lock compensate for stretching of the bead lock upon enlargement. The edges of the bead lock atthe axial sides of the chamber are undercutfor flexibility while the walls (70,71) of the chamber are specially coated to maintain the seal. The bead lock incorporates interconnected metallic segment side plates (76, 77) which not only limit the stroke of the moving seal but insure stability axially of the bead and provide the desired concentricity. The bladder is pressurized through an annular air passage system and sealed conduit (137) extending radially through the pressure chamber inside the bead lock.