Which component must be flattened while correcting a loose thermostat shaft on a FD series thermostat?

When correcting a loose thermostat shaft on a FD series thermostat, flattening the cup-shaped plate is essential as it directly impacts the shaft's ability to make proper contact with the thermostat's internal mechanism. The cup-shaped plate serves as a pivotal interface between the thermostat shaft and the internal components that regulate the temperature. When this part is properly flattened, it ensures that the shaft maintains a secure connection, allowing for accurate temperature readings and adjustments.

This action helps to eliminate any play or looseness that may disrupt the function of the thermostat, ultimately leading to better performance in maintaining the desired temperature for the appliance it controls. In contrast, altering the thermostat knob, body, or adjustment screws would not achieve the same effect since they do not play a direct role in ensuring the secure fit of the shaft within the thermostat.