Plug electrical knock-outs. Molded in silver gray color to resemble galvanized steel. Install quickly with fingertip pressure (no tool required). Economical. Multiple trade sizes fit 1/2" to 2" knock-out holes. Lock firmly in boxes up to 1/8" thick, except for Part Number 4002 (KOS 2) and Thick Panel Knock-Out Seals (KOST's), which lock firmly in boxes up to 1/4" thick. Locking fingers in fractional increments up to the maximum panel thickness. Approximate Head Thickness: .062" (1,6 mm). Thickness may vary. To adapt a Knock-Out to accept a Heyco Strain Relief, see Heyco Knock-Out Adapters.