Generally speaking,for measurements, words such as height, weight, length, diameter etc. do not require the plural.
The diameter of the two ropes or two ropes. The length of the chests. The weight of the whales.
However, if you are making a general statement about diameters, you would use the plural.
Diameters of pipes are not easy to measure.
But: The diameter of those pipes is engraved into them.
Just one example:
Sizing Pipes
The diameter of the pipes in the irrigation design, along with the size of the valves and system components, has a direct effect on the pressure and velocity of the water moving through the system, which, in turn, affects the performance of the outlets and the longevity of the pipes and other components. As pipes are placed in the design, they are automatically sized based on velocity, if the Min Diameter Not Exceeding Velocity Limit option is selected.diameter of the pipes