a core assembly is formed by fabricating a first core , molding a destructible plastic layer around the first core , and molding a second core around the plastic layer . the plastic layer is interlocked .. . iadd . with . iaddend . an irregular outer surface of the first core , and the second core is interlocked with an irregular outer surface of the plastic layer . the core assembly can be used for casting engine cylinder heads to eliminate the need for gluing cores together and the accompanying formation of fins within fluid conducting passages .