Abstract: In this piston structure with a heat insulated combustion chamber, a combustion chamber structure is fixed to a piston body stably and firmly by a pre-stressed holding member. The combustion chamber structure which constitutes a combustion chamber provided on an outer circumferential part of a lower portion thereof with a flange made integral therewith, and which comprises a heat resisting material, is fixed by a holding member fused to the piston body. After the piston body and holding member have been welded, the holding member is pre-stressed by a press so that the fixed condition of the combustion chamber structure with respect to the piston body during an operation of the engine is stabilized.

Abstract: The present invention provides a pre-combustion chamber-type engine, which optimizes the relation between the heat capacity of the pre-combustion chamber structure that forms the pre-combustion chamber and the displacement per cylinder and thereby improves the net average effective pressure and fuel efficiency. In this pre-combustion chamber-type engine, which has the pre-combustion chamber structure 15 installed in the cylinder head 3 so that it is surrounded by a heat insulating layer, the ratio of the heat capacity of the pre-combustion chamber structure to the displacement per cylinder is set to 0.35 cal/.degree.C..multidot.cm.sup.3 or less. Alternatively, the ratio of the heat capacity of the pre-combustion chamber structure to the inner surface area of the pre-combustion chamber is set to 0.55 cal/.degree.C..multidot.cm.sup.2 or lower.