A shim is sometimes installed in the groove of a piston ring to enhance performance, particularly in older designs before the introduction of the alfin ring in heavy vehicle diesel engines. This thin metal strip is placed in the inner top portion of the first groove during manufacturing, serving a specific function distinct from standard piston rings. The discussion highlights confusion regarding its application, with inquiries about its use in various engine types and the materials involved. It also raises questions about the shim's sealing capabilities, wear patterns, and operational conditions. Overall, the shim's historical significance and its role in piston design are emphasized.