A SUS303 stainless adapter linking an AN3 line to an AN4 line. Mixed sizes are common on real builds — an AN3 brake hose meeting an AN4 hard line, or a small hydraulic circuit assembled from parts sourced at different times — and this adapter makes the transition in one component instead of a stack of two. Both ends use the 37° AN flare seat, sealing on the cone rather than the thread, so keep sealant well away from it. SUS303 stainless handles the brake fluid, water and grit that this part of a vehicle lives in. Note that a size step is also a bore step, so place it where flow is not critical rather than in the middle of a long run. Bleed thoroughly after fitting and pressure-check the joint before driving.