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The boring of valve seats of internal combustion engines is done approximately 25 million times per year in Brazil. Excessive vibrations during this operation are common and can adversely affect dimensional and geometrical tolerances of the valve seats. The sealing of the combustion chamber is prejudiced, resulting in loss of power and higher emission of pollutant gases, harming the environment. The raw material of the valve seats is sintered steel of 370-410 HB hardness and is machined with expensive polycrystalline cubic boron nitride blades. In order to avoid the generation of excessive vibration or chatter during machining, operators reduce cutting speed and feed rate, which consequently causes reduction of productivity. In this context, investigation of the process was performed with the objective of minimizing the vibrations during the valve seat boring through changes in the tool holder design and in geometry of the cutting blades. The proposed modifications resulted in a remarkable reduction in magnitude of the resultant cutting force, vibrations, and roundness deviations of the valve seat. Hence, the sealing between the valve and the seat was improved and the emission of gases from the motor could be reduced, with gains in efficiency. The results also showed that the cutting blade edge must have a 60-μm honing, and the best cutting parameters are in the range: cutting speed 80-100 m/min and feed rate 0.04-0.08 mm/rev/tooth. [ABSTRACT FROM AUTHOR]