A tire building core assembly includes a shell assembly is formed by a plurality of shell segments and includes a central spindle-receiving throughbore. Two spindle units meet in axial alignment within the shell assembly throughbore and retain the shell segments in an expanded configuration forming a toroidal tire building surface. An open socket may be formed in a protruding end of one or both spindle units, the socket configured to couple with an end of an external arm mechanism. A latching mechanism moves between engaged and disengaged positions for releasably couple the one spindle unit to the second spindle unit within the shell assembly throughbore.