Rock and Swivel 3318G: Floor to top of mechanism with casters, base, gas cylinder, and swivel tilter mechanism-Low is 15 1/2", high is 18".
Simply insert without tools the gas cylinder into the center hole of the 4 leg chair base. Just sitting in the chair several times will make extremely tight and require tools to remove.
A mounting board may be necessary to install SKU 3318 or any other size mechanism.