This copper sealing washer sits between the fuel-system banjo fitting and the mating surface to create a leak-tight joint. It deforms under the specified torque to fill microscopic irregularities in the sealing faces, preventing air ingress into the fuel line and fuel seepage at the connection. A crushed or re-used washer loses its ability to seal, allowing air to enter the system, which causes hard starting and erratic idle. Because the washer is dimensionally matched to the original fitting groove, it compresses to the correct thickness without over-stressing the banjo bolt. Replace the washer every time the fuel line is disconnected; do not reuse.