When installing a new piston onto the connecting rod, what is the significance of the 'IN' mark or arrow stamped on the piston crown?

1. It must face toward the intake valve (intake manifold side) of the engine.
2. It must point toward the inside (crankcase center) of the engine.
3. It indicates that the piston should face toward the exhaust port.
4. It indicates the direction of rotation of the engine flywheel.