Abstract: A breaker core for use in metal casting is described. The core is located in a casting mould between a riser and the cavity corresponding to the casting. The core consists of a base having an aperture (which defines a neck between the casting and the solidified metal in the riser) and means for frictional engagement with a casting riser lining sleeve or with a casting riser cavity. The base may have a recess which receives a projection extending from the sleeve or an upstanding lip at or somewhat spaced from the edge of the base for engagement with an inner or an outer wall of the riser sleeve.