Cup seals are two-piece metal tamper-evident devices that lock permanently through a 360-degree multi-point locking ring, and can only be opened by physically destroying the seal. Unlike plastic pull-tight or strap seals, cup seals are designed for the most sensitive and high-value closures, where any attempt at unauthorized access must be immediately visible and impossible to conceal. AC&M cup seals are used across nuclear regulatory programs, hazardous material handling, precision instrumentation, and industrial processing systems where standard tamper evidence is simply not enough.

The seal body is available in galvanized steel, copper, or brass, providing durability against physical damage, extreme temperatures, and corrosion. The result is a seal that holds its integrity long-term and signals unauthorized access the moment it is disturbed.

How the 360-Degree Locking Ring Works

The defining feature of an AC&M cup seal is its locking mechanism. The seal consists of two metal pieces. When the top is pressed onto the bottom, hard sharp teeth on the interior locking ring engage around the full inside circumference, creating a multi-point grip that cannot be released without tools. Once locked, the pieces cannot be separated without visibly deforming or destroying the seal body.

This is what separates cup seals from most other tamper evident security seals. A standard indicative seal can sometimes be defeated without obvious visible damage if the right tools are used. A cup seal cannot. The destruction of the seal IS the evidence. There is no way to open it, reseal it, or make it appear undisturbed.

Wire Sealing: Securing Openings of Any Size

The first configuration of the AC&M cup seal is wire sealing. In this application, a twisted wire of any required length is threaded through two holes in the bottom of the seal body and tied together inside the housing before the top is locked. The wire can be attached through any opening of 1mm or greater, making it one of the most versatile tamper seals for irregular or hard-to-reach closure points.

This configuration is commonly used across demanding environments where standard seal types are impractical. Wire seals used in combination with a cup seal body provide a level of security that no plastic or cable seal can match in these contexts. Typical applications include:

• Hazardous material containers requiring regulatory-grade tamper evidence
• Nuclear materials in storage and during transport, used by governmental and international nuclear regulatory inspection bodies
• Precious metals storage and transport
• Meters and measuring equipment in chemical, food processing, and petrochemical refinery systems
• Valve and switch lockouts in industrial processing facilities

The wire is concealed and held within the sealed housing, meaning it cannot be cut and re-tied to simulate an intact seal. Any interference with the wire requires destroying the cup seal itself.

Screw and Bolt Protection: Sealing the Fastener Itself

The second configuration addresses a specific and often overlooked vulnerability: the exposed fastener head. In many regulated or high-value applications, access to a device is controlled by screws or bolts, but the fastener heads themselves remain accessible. Specialty screw driver bits are now widely available commercially, meaning a security screw head alone offers limited protection.

AC&M cup seals solve this directly. A single hole in the bottom of the seal allows it to be placed over a screw or bolt. The fastener is tightened normally through the seal base, then the top snaps into place, enclosing the fastener head entirely. From that point, the screw cannot be accessed or turned without destroying the seal. Because the cup seal is solid metal, it cannot be opened and then reattached with glue or adhesive to simulate an undisturbed seal, unlike some plastic alternatives.

AC&M offers a range of standard hole sizes compatible with both inch and metric fasteners, covering a wide range of common head and screw diameters. Sizes range from small 3/8" diameter cups to large 1" diameter cups, with cap heights available in 5/16" or 7/16" for taller bolt heads. Browse the full range of cup seals for screw and bolt protection to find the right fit for your application.

Common applications for screw and bolt cup seals include:

• Calibrated test and measurement equipment
• Racing engines after passing qualifying inspections
• Metering devices and utility infrastructure
• Electronics, servers, and controls
• Switching equipment and industrial automation panels

For utility metering specifically, cup seals are one of the most reliable tools for preventing meter tampering and utility theft, since the seal must be destroyed to access the fastener securing the meter housing.

Customization and Serialization

Every AC&M cup seal can be stamped with letters, serial numbers, logos, or other markings on both the top and bottom pieces. Matched numbering on both halves adds an additional layer of control, since any replacement of one piece with an unstamped duplicate becomes immediately detectable during inspection.

This serialization is essential for applications that require a documented security seal verification and inspection program. Operators can record the seal number at the point of application, then verify that the same numbered seal remains intact at each subsequent inspection. If the number does not match or the seal shows any sign of distortion, a breach is confirmed.

Seals can be supplied in bulk for cost savings or packed in numerical order in individual boxes for controlled sequential application under strict protocols. The combination of numbered serialization and physical destruction-only opening makes cup seals one of the most auditable tamper-evident options available.

Internal Traps: An Additional Layer of Evidence

Some users of AC&M cup seals take tamper detection a step further by placing an internal trap inside the seal before locking. A tamper trap refers to any object or material placed inside the seal housing that would shift, deform, or change appearance if the seal were opened and reclosed.

Even if someone were to find a way to open the cup seal without causing visible external damage, the internal trap would reveal the breach. This two-layer approach, combining the physical destruction of the seal body with concealed internal evidence, is used in some of the most sensitive inspection and regulatory programs AC&M serves.

This technique is an option, not a requirement. Out of the box, the metal seals of the cup seal type already provide a very high level of tamper evidence. The trap method simply extends that capability for applications where absolute certainty is required.

When to Choose a Cup Seal Over Other Seal Types

Most tamper-evident applications are well served by plastic strap seals, pull-tight seals, or cable seals. Cup seals are not a general-purpose product. They are the right choice when one or more of the following conditions apply:

• The closure point is a screw or bolt head that must be fully enclosed
• The application involves a wire or cable attachment through a small or irregular opening
• The regulatory or operational environment requires evidence of tampering that cannot be simulated or concealed
• The seal must withstand extreme temperatures, corrosion, or physical stress over a long period
• The application is subject to governmental, nuclear, or international inspection requirements

For a broader view of how different seal types compare and which applications each is suited for, the Guide to Tamper Evident Security Seals provides a comprehensive reference. Following best practices for sealing programs ensures that whichever seal type is selected, it is applied, documented, and inspected in a way that holds up under audit.

The Most Demanding Applications Deserve the Most Capable Seal

When the standard options are not enough, the American Casting & Manufacturing cup seal is built for what comes next. Its 360-degree locking ring, metal construction, dual-configuration design, and serialization options make it the most capable tamper-evident seal AC&M manufactures. Government inspection programs, nuclear regulatory bodies, and precision instrument operators have relied on it for decades.

If you are evaluating cup seals for a specific application, AC&M can provide samples and expert guidance to help confirm the right size, material, and configuration for your needs.