3/8 inch primer bulb. E.P.A. / C.A.R.B. compliant low permeation primer bulb. Double check valves to hold prime longer. UV inhibitors protect from prolonged exposure to sunlight. Oversized bulb works for most outboards. Large I.D. for highest flow rate. 3/8" inside diameter Double check valve E.P.A. / C.A.R.B. compliant UV inhibitors protect from sunlight Meets U.S.C.G./ABYC/EPA Req Oversized bulb works for most outboards Large I.D. for highest flow rate