A fluid comparator assembly which is clamped to the floor of a cotton harvesting unit and includes an arm for contacting the top of a spindle. The arm is operably connected to a fluidfilled cylinder communicating with a gauge. As the picker bar drum is rotated and a different spindle contacts the arm, any deviation in spindle height will be indicated by a change in fluid column height on the gauge. The cross-sectional area of the cylinder is much greater than that of the gauge column so a small deviation in spindle height results in a large, easily readable change in fluid column height. Each spindle bar is shimmed so that the maximum deviation as indicated by the change in column height is within the desired tolerance.
