Product review (1 stars): OK I am not sure what is 3 1/8" about ...
OK I am not sure what is 3 1/8" about these devices, they clearly measure 3 5/16" if you tun them over and measure the diameter.  So I am a bit surprised about the people who claim they installed them in a 3 1/8" sink opening.  My sink measured 3 1/8" and would not take one of these.