The auxiliary combustion chamber of a four-cycle stratified-charge piston engine is divided into a primary cavity and a secondary cavity both formed in an insert liner received in the cylinder head. The primary cavity receives a relatively rich mixture which passes into the secondary cavity, and the main combustion chamber receives a relatively lean mixture. A spark plug communicates with the primary cavity. A long torch passage connects the primary cavity with the central portion of the main combustion chamber, and one or more short torch passages connect the secondary cavity with a peripheral portion of the main combustion chamber. The purpose is to reduce emission of NO.sub.x without adversely affecting emissions of CO and HC, and to improve fuel economy.