Is the piston of a reciprocating machine subject to an upward buoyant force during normal operation?

Actually, this is not an issue of upward floating; when at rest, in addition to the support ring, the piston rod sometimes also experiences some force. The two points where the support ring and the piston rod are under stress are not necessarily on the same straight line; in this case, the piston hangs downward due to the effect of gravity. During operation, as the piston rod moves back and forth, the point of support for the piston rod keeps changing, and the magnitude of the torque also changes continuously; when the torque is at its smallest, it seems as though the piston rod rises. The actual situation is related to the design of your piston support rings, the installation dimensions of the piston, and the size of the piston itself. Under normal conditions, the piston is surrounded by the support rings and maintains a safe distance from the cylinder liner.

The piston ring is subjected to an upward or downward force (the forces in the two symmetric cylinders are opposite), but it is not buoyancy; rather, it is a component of the force exerted by the crank on the slider. If the engine rotates counterclockwise (as viewed from the piston side), the right cylinder is subjected to an upward component force at all times, whether during intake or exhaust, while the left cylinder is subjected to a downward component force.

The situation you mentioned occurs during the operation of crosshead compressors. In most crosshead compressors, such upward or downward forces can only act on the crosshead, resulting in stress on what is known as the upper slide or the lower slide of the crosshead; the piston is not affected by this.

Buoyancy exists only in fluids; some experts rely on their imagination and try to use what they have learned to conduct scientific analysis and judgment