Monobloc piston having open floor

1. A piston assembly comprising:

upper and lower piston crown parts fabricated of steel and united by at least one friction weld joint and having an oil cooling gallery bounded by an inner side wall, an outer side wall, a top wall and a bottom wall;

said bottom wall being fabricated of one piece with said lower crown part and including at least one oil passage slot in said bottom wall that has a length in the circumferential direction that is greater than a width in the radial direction.

2. The piston assembly of claim 1 wherein said inner wall includes said friction weld joint and said outer wall includes a weld joint.

3. The piston assembly of claim 2 wherein said weld joint of said outer wall comprises a friction weld joint.

4. A piston assembly, comprising:

an upper piston crown part fabricated of steel;

a lower piston crown part fabricated of steel separately from the upper piston crown part,

said upper and lower crown parts united by at least one friction weld joint and including an oil gallery boxed by a top wall, a pair of radially spaced inner and outer side walls and a bottom wall;

said bottom wall being fabricated as one piece with said lower crown part;

a pair of pin bosses and a piston skirt formed as one piece with said lower crown part;

said bottom wall being formed with an elongated opening that is of sufficient size and in position to receive a stream of oil from an oil injector positioned below said piston assembly over a majority of the stroke of said piston assembly as is reciprocates up and down within a piston cylinder of an engine in operation.