GITD (glow-in-the-dark) silicone gaskets O-ring Some sizes you will need to stretch-to-fit AND is a little thicker than the original O-ring Emisar D4 or D4sv2: 35mm x 1.5mm (stock O-ring is 35.5 x 1mm) Emisar D4v2, D4k or Kr4: 27mm x 1.5mm (stock O-ring is 1.5mm so will have a .5mm gap between bezel and head) Lumintop FW3A/FW1A: 22mm x 1