A piston assembly for internal combustion diesel engine having a piston diameter of 160-650 mm, the piston assembly including a top part, a body part and a gudgeon pin connectable to each other, the top part defining, when installed in a cylinder, the piston side of a combustion chamber, and the body part having an aperture for a gudgeon pin, bosses for distributing forces between the piston and the gudgeon pin, the body part having an interior, an outer surface and operable connecting surfaces, the gudgeon pin for pivotally connecting the piston to a connecting rod, is configured to be assembled and lockable by locking elements to the body part aperture in a given rotational orientation such that there is provided a conduit for supplying cooling oil from the connecting rod via the gudgeon pin to the bosses and to a cooling gallery between the top and body parts.