Manganese bronze. Comes with special shoulder for ease of installation. Takes OEM valve seals. Shortened to accept valve lifts up to .600″. Softer and less durable then C630 bearing bronze but will allow a lower cost and reaming to size is quick and easy. Finish can be done with simple tools like a ball hone and an electric drill. Rapid heat transfer and excellent lubricity. Note: Special Manganese reamers & hones are available. Note: Standard non-oversize dimensions (for reference only) are 0.5620″ O.D. (14.27mm); .3085″ I.D. (7.84mm); 2.100″ OAL (53.34mm).