The Bonded Valve Seat System is the latest technology to realize drastic reduction in valve temperature in SI engines characterized by the good thermal conductivity of extremely thin valve seats bonded directly on the aluminum cylinder head. A unique and highly rationalized resistance bonding technique was developed to maintain adequate bonding strength and positioning precision in a short bonding period of around one second. Engineering data on optimization of bonding-section geometry, valve seat material and the surface treatment and bonding parameters were presented and discussed regarding the mechanism. The geometry of the bonding section of the cylinder head was optimized by FEM analysis so that the aluminum material should deform to embed the valve seat ring with the action of expelling the surface contamination and the oxide film. The bonding facility was modified so that the electrode axis should move flexibly according to distortion of the cylinder head during bonding. A new non-destructive inspection methodology was developed to guarantee the sound bonding status in mass production.The effect of bonded thin valve seats on temperature distribution of a cylinder head assembly was studied using FEM analysis as well as engine bench testing. Results showed that the valve temperature in the bonded valve seat system was lower by as much as at least 50°C compared to that in the conventional press-fit-type valve seat system. Endurance testing using several automotive engines was carried out to confirm the reliability. The testing results revealed the engines with the system had passed the endurance criterion. Metallurgical study of the bonded section showed neither brittle intermetallic compounds nor destructive faults generated at the bonding interface.It is concluded that the bonded valve seat system is effective in reducing the engine valve temperature and has adequate reliability for application to high-volume engine manufacturing.