Flow Security Systems Vented Locking Fuel Tank Cap 2" Male Thread — Case‑Hardened Carbon Steel, Built‑In Vent Valve

The Flow Security Systems FSS-377 is a vented, locking 2" male-thread fuel tank cap engineered to stop unauthorized access, siphoning and vandalism while allowing safe pressure equalization.

This heavy-duty locking cap and adapter set is built for mobile transfer tanks and above-ground commercial fuel tanks that use a 2" male standard thread. CNC-machined from carbon steel, case-hardened for resistance to drilling and finished with a sealed gasket and a built-in venting valve, the FSS-377 combines mechanical protection with functional venting so tanks can breathe without compromising security.

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

The cap locks to a threaded adapter using a shackle-style padlock (5/16" diameter required), creating a physical barrier that deters casual theft and makes siphoning significantly harder. Case-hardening and precision machining increase resistance to cutting and drilling attempts.

• Requires any 5/16" shackle padlock (not included) to lock cap to adapter
• Case-hardened carbon steel resists drilling and cutting
• CNC-machined tolerances minimize pry points and ensure secure fit

Will the cap allow my tank to vent safely while remaining locked?

Yes. The FSS-377 includes an internal valve that equalizes pressure and vacuum automatically so the tank breathes without removing the cap. Venting specifications are engineered to open at low differential pressures, reducing stress on hoses and fittings during normal use.

• Built-in valve vents at approximately 1.5 psi to relieve pressure
• Vacuum relief set at approximately 1.5 oz to prevent tank collapse
• Brief vent cycle may allow minimal spillage during rapid fill events

How do I install and maintain the FSS-377 on my tank?

Installation is straightforward: the adapter threads onto the 2" male tank opening and is tightened with a wrench; a thread locker can be applied for a more permanent installation. The cap attaches to the adapter and is retained with the padlock; a 10" stainless retaining chain keeps the cap accessible during filling and can be removed if needed for larger nozzles.

• Fits standard 2" male threaded tanks—no special tank modifications
• Adapter secured with wrench; optional thread locker for permanence
• 10" stainless steel chain retains cap during fill-ups; removable

What materials and construction make this cap durable?

The FSS-377 is manufactured from high-quality carbon steel and finished with case hardening to increase surface toughness. Precision CNC machining ensures consistent sealing and fit, and an internal gasket prevents leaks under normal operating conditions.

• Carbon steel construction for structural strength
• Case-hardened surfaces to resist tampering and wear
• Internal gasket creates a sealed system to minimize spillage

Who is this for: The Flow Security Systems FSS-377 is aimed at fleet managers, mobile fuel vendors, construction sites, agricultural operations and commercial facilities that store fuel in above-ground tanks or transfer tanks and need a rugged, lockable vented cap. It’s ideal for anyone seeking a compact, mechanically robust solution to reduce fuel loss and unauthorized use without altering tank plumbing.