Flow Security Systems vented locking fuel tank cap — 2" male thread, hardened steel lock, built-in vent

The Flow Security Systems FSS-377 is a vented, locking 2" fuel tank cap engineered to secure transfer and above-ground commercial tanks against theft, tampering, and vandalism while allowing controlled pressure equalization.

Built for fleet managers, jobsite supervisors, and anyone who stores fuel in mobile transfer tanks or above-ground commercial tanks, the FSS-377 combines a hardened steel locking cap with a precision adapter and an internal vent valve. It provides a rugged, reusable solution that fits standard 2" male threads and works with common 5/16" shackle padlocks (padlock not included). The design emphasizes durable materials, secure retention, and an integrated venting mechanism to keep tanks functioning safely under normal pressure changes.

How does the FSS-377 prevent unauthorized fuel access?

The FSS-377 uses a heavy-duty, case-hardened carbon steel cap and adapter that lock together with a user-supplied 5/16" shackle padlock, creating a tamper-resistant barrier. CNC machining ensures a precise, tight fit that resists prying and drilling.

• Case-hardened carbon steel construction resists cutting and drilling.
• CNC-machined parts for consistent locking engagement.
• Accepts standard 5/16" shackle padlocks (not included) for flexible security options.
• Internal gasket seals tank contents to reduce spills during normal use.

Will it fit my tank and how is it installed?

The FSS-377 is made to thread onto any tank with a 2" male standard thread. Installation is straightforward: attach the adapter to the tank with a wrench and secure the cap with a padlock. For a permanent fit, a thread locker can be applied to the adapter threads.

• Compatibility: all 2" male threaded tanks.
• Adapter installs with a standard wrench; no special tools required.
• Optional thread locker for permanent installation.
• Cap attaches to adapter and is locked with a 5/16" shackle padlock.

Does the cap allow tanks to vent and avoid pressure issues?

Yes. The FSS-377 includes a built-in valve that equalizes pressure and vacuum to protect tank integrity. The valve opens at specific thresholds to vent pressure or relieve vacuum without compromising security.

• Vent opens at approximately 1.5 psi to relieve excess pressure.
• Vacuum relief activates at about 1.5 oz to prevent tank collapse.
• Designed to minimize spillage; brief venting may allow small amounts of overflow.
• Valve is integrated into the cap for continuous protection while locked.

What durability and convenience features does it offer?

The FSS-377 is engineered for heavy-duty, long-term service with components that resist wear and vandalism. Practical touches like a removable stainless steel retaining chain and an interior sealing gasket improve daily usability during fueling.

• CNC-machined carbon steel for long service life and consistent fit.
• 10" stainless steel retaining chain keeps the cap nearby during refueling and is removable for oversized nozzles.
• Interior gasket seals tank contents and reduces spills between fills.
• Hardening treatment improves resistance to damage and tampering attempts.

Who is this for: The FSS-377 is intended for businesses and individuals responsible for mobile fuel transfer tanks, construction sites, agricultural operations, and commercial above-ground storage where fuel theft, siphoning, or vandalism is a risk. It suits anyone who needs a rugged, threaded locking cap that balances theft protection with necessary venting and everyday practicality.