hydraulic jack valve release screw knob comes in two sizes to either the jack screw with a 3/8" and 7/18" diameter valve release screw It is machined from a solid bar of 6061 aircraft quality aluminum, The knob also has a longer knurled section ,for extra grip. It slides on over the release screw, turns 1/4 turn then secured in place with a set screw (included) from the end of the knob. You will need to use a 5/32 hex key (Allen wrench) This particular part is designed for the Harbor Freight Hydraulic Bottle Jack used in the 6, 12 & 20 ton arbor presses but will work on any bottle jack with a 3/8 and 7/18" diameter valve release screw having a DO NOT USE WHEN LIFTING VEHICLES: ONLY TO BE USED ON HYDRAULIC PRESS APPLICATIONS!