GROCO 4" Bronze High Speed Thru-Hull Fitting w/Nut

The GROCO 4" Bronze High Speed Thru-Hull Fitting with Nut is the marine plumber's answer to maximizing raw water flow without sacrificing hull integrity or creating drag penalties. Built for anglers, cruisers, and serious boaters who demand reliable through-hull performance, this 4-inch fitting serves as the backbone of any raw water cooling system, washdown rig, or live well setup. Whether you're running a saltwater engine cooling loop or pressurized deck wash, a properly sized high-speed thru-hull fitting makes the difference between an efficient system and a bottleneck that costs you RPMs and reliability.

Machined from C83600 bronze—a corrosion-resistant alloy engineered specifically for immersion in saltwater and brackish environments—this fitting resists dezincification and maintains structural integrity across years of exposure. The high-speed design reduces turbulence and pressure drop compared to standard thru-hull geometry, allowing more water volume at lower velocity and less drag on your hull. NPSM/NPT combination threading accommodates both straight pipe (NPT) and machinist's parallel (NPSM) adapters, giving you flexibility in plumbing configurations. The included nut ensures secure installation and sealed fastening below the waterline, critical for preventing the slow leaks that can compromise bilge integrity over months and seasons on the water.

This 4-inch fitting is ideal for mid-size to larger sportfish boats, trawlers, and cruising sailboats where raw water demand exceeds what smaller 2- or 3-inch fittings can deliver. Installation requires a proper 4-inch core hole below the waterline, reinforcement backing plates, and marine sealant rated for saltwater use—not a weekend job for novices, but absolutely achievable with a through-hull saw and proper technique. Anglers who've invested in redundant cooling systems or enhanced live wells will recognize the 4-inch bore as the threshold where system performance becomes genuinely responsive to engine load and ambient water temperature changes.