A brass olive insert sized for AN4 PTFE hose ends, sitting between the PTFE liner and the stainless braid where it is compressed to make the seal. AN4 sees more service than any other size in a typical build, covering turbo oil feeds, small fuel lines, gauge feeds and clutch circuits, so these are the olives you get through fastest. The brass is deliberately softer than the components around it, deforming to fill the gap without damaging the PTFE tube. Fit a new one every time you assemble a hose end, including on reused fittings, as the original olive will have already been crushed to a shape that no longer seals against fresh hose.