Flow Security Systems locking vented fuel tank cap FSS-377 — 2" male thread, hardened carbon steel, pressure-equalizing vent

Designed for fleet, construction, agricultural and commercial fueling applications, the FSS-377 combines mechanical strength with a pressure-equalizing valve to protect stored fuel while allowing safe venting. CNC-machined carbon steel components and a case-hardened finish provide resistance to forced entry and wear. The cap fastens to a threaded adapter and uses a standard 5/16" shackle padlock (not included) to lock, while an internal gasket and retaining chain control spills and keep the cap accessible during refueling.

How does the FSS-377 stop fuel theft and tampering?

The cap replaces standard non-locking covers with a robust, lockable steel assembly that resists drilling and brute force. Locking the cap to the adapter prevents unauthorized removal, and the solid construction makes siphoning and tampering far more difficult.

• Case-hardened carbon steel resists drilling and cutting attempts
• Uses any 5/16" shackle padlock to secure cap to adapter
• Adapter threads into tank and can be permanently fixed with thread locker
• Retaining chain prevents cap loss during fueling operations

Will the venting feature affect tank performance or cause spills?

The FSS-377 includes a built-in relief valve to equalize pressure and vacuum at controlled setpoints, helping prevent tank deformation or siphoning-related vacuum issues. Although the gasket seals the opening, brief venting at set pressures can release a small amount of vapors or fluid under extreme conditions; routine operation should not produce noticeable spillage.

• Valve opens to vent pressure at approximately 1.5 psi
• Vacuum release set to about 1.5 oz to prevent tank collapse
• Internal gasket reduces leakage and controls drips
• 10" stainless steel chain holds cap during fills and is removable if needed

Which tanks and locks are compatible with this cap?

The FSS-377 is engineered to fit any tank with a 2" male standard thread, making it suitable for many transfer and above-ground commercial fuel tanks. The cap secures through a simple shackle padlock connection, so you can use widely available 5/16" shackle padlocks from your preferred security brand.

• Fits standard 2" male threaded tank openings
• Accepts any padlock with a 5/16" diameter shackle
• Adapter installs with a wrench for a tight, leak-resistant fit
• Thread locker can be applied for permanent installation

How durable is the FSS-377 for field and commercial use?

Built for continuous, heavy-duty service, the cap and adapter are CNC machined from high-quality carbon steel and case-hardened to increase surface hardness and longevity. These manufacturing choices reduce wear from repeated use and increase resistance to common attack methods used by fuel thieves.

• High-grade carbon steel construction for structural strength
• CNC machining ensures precise fit and smooth operation
• Case-hardening improves resistance to cutting and drilling
• Stainless steel retention chain provides corrosion resistance

Who this is for: the FSS-377 is intended for fleet managers, site supervisors, equipment owners and anyone responsible for safeguarding stored fuel on mobile transfer tanks or above-ground commercial tanks. If you need a straightforward, heavy-duty locking solution for a 2" male-threaded tank that vents safely and withstands regular field use, the Flow Security Systems FSS-377 offers a durable, easy-to-fit option.