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 1.5mm (stock O-ring is 22.5 x 1mm)
FWAA: 19mm x 17mm x 1.0mm
FWAA: 19mm x 19mm x 1.5mm (slightly thicker and brighter will have gap between bezel and head)
18mm for FWAA WITHOUT STOCK GLASS OPTICS ONLY (THICKER O-RING BRIGHTER GLOW)
18mm for FW1AA fits slight gap from the bezel to the body
PLEASE READ FOR FWAA! THERE IS "2" FWAA GITD O-ring ONE is thicker than the other. Thicker one the stock glass needs to be removed if you want the bezel flushed with the body, the thinner one is about the same size as the stock O-ring but its thinner, so it doesn't glow nearly as bright
