Mercury – Fuel Tank Diurnal Cap with In-Tank Retainer | Fuel System | 36-8M0064067

Overview

Portable fuel tanks take a beating—sun, movement, pressure changes—and the cap is often the first thing to quit on you. When it stops sealing right, you get fuel smell, pressure issues, or a tank that bulges in the sun. This Mercury diurnal cap is the correct replacement for the newer 3.2- and 6.6-gallon tanks. It threads on cleanly, vents the way these tanks are meant to, and uses an in-tank retainer so the cap stays put while you’re fueling. If your cap’s been cracked, loose, or just plain annoying, this one brings the tank back to behaving itself.

Key Features

 • Diurnal Venting – Manages pressure changes from hot days and cool nights.
 • Stainless Chain & Retainer – Keeps the cap attached so it doesn’t end up in the drink.
 • EPA-Compliant – Meets 2011 pressed-tank requirements for portable systems.
 • Tight, Reliable Seal – Helps cut down fumes and odd tank pressure issues.
 • OEM Fit – Made specifically for Mercury and Quicksilver fuel tanks.

Engine & Boat Compatibility

 • 3.2-gallon tanks: 8M0054600, 8M0047598
 • 6.6-gallon tank: 8M0045691
 • Mercury / Quicksilver portable fuel tanks

Performance & Practical Benefits

A proper cap makes the tank behave the way it should. No swelling in the heat, no collapsing at dusk, and no raw fuel smell when you open a storage locker. The venting is tuned for marine use, so the tank breathes just enough without dumping fumes. And the retainer is a simple perk—keeps the cap from rolling around the deck or disappearing overboard.

Why Buy OEM

OEM caps match the threads and sealing surface exactly. They vent correctly and keep pressure where the tank expects it to be. Aftermarket caps often look the part but don’t regulate pressure right, which can lead to hard starts or starving the engine. With an OEM cap, the tank works the way it should.

FAQ

Q1: How do I know my old cap needs replacing?
A: Fuel smell, loose or cracked plastic, or a tank that swells or sucks in with temperature changes.

Q2: Do I need tools to install it?
A: No—just twist off the old one and thread the new cap on.

Q3: What does the diurnal vent actually do?
A: It lets the tank breathe slowly as temperatures change so pressure stays controlled.

Q4: Will this help with tank over-pressurizing?
A: Yes—a proper diurnal cap keeps pressure in check.

Q5: Is the stainless chain really useful?
A: Definitely—keeps the cap with the tank while fueling and saves you from losing it overboard.