PURPOSE: The mold which comprises fitting the main core and sub-core of the specific shapes in the two-split outer dies having a circular space in cross section and which is capable of simultaneously casting the piston provided with a narrow insulation gap between pin-boss parts and the inside circumferential part of skirt part.
CONSTITUTION: A main core 5 is inserted into the space S of two-split outer molds 1 and its pin-boss part forming recesses 9 are opposed to the through-hole 3 of the outer dies. Thence, a sub-core 12 is inserted into the dies through the through-hole of the outer side and the end face 14 of its partition part 13 is contacted to the plate material 10 coiled and fixed to the skirt part inside surface forming part 7. At the same time, the positioning pin 16 provided to the end of the pin-hole-forming protruding part 15 into the hole 17 of the main core. If molten metal is poured into the cavity of the resultant mold, a casting 21 is obtained and the insulation gap S4 equal to the thickness of the plate material 10 is formed between the pin-boss parts 18 and the inside circumference of the skirt part 20.