Mercury - Outer Helm Extension Harness 20-Foot Extension 84-8M0159752. OverviewThis 20-foot extension gives the outer engines on triple- and quad-engine rigs the reach they need when the standard harness falls short. Wide transoms and long rigging runs can make things tight, and this harness keeps you from forcing sharp bends or stretching cables. It lets the wiring take a smoother path so everything feels calmer at the helm. It's a simple add-on that makes big multi-engine layouts easier to rig