Choosing the Best Metal-to-Metal Seal Design


In any solid block of metal, where multiple ports have been drilled to allow for cross-channel fluid or gas conveyance, proper sealing of the drilled port is critical to performance. When evaluating seal options engineers must consider much more than the potential for leakage, but also installation, cleanliness requirements and materials. Proper metal-to-metal seal selection ensures reliable performance, long-life and lower total costs.

Many common metal-to-metal seals – such as screw plugs, ball bearings and cup plugs – are not well-suited for today’s precision transmission systems. Metal-to-metal seal selection has become a more complex decision because of hydraulic fluids with unique viscosities, very thin port wall thicknesses, and materials with low elasticity moduli that are being used more frequently.

Expander seals are an increasingly popular metal-to-metal seal option. Their unique design features a serrated sleeve and internal element, such as a ball or mandrel. When force is applied, the internal element expands the serrated sleeve to engage the base, forming a reliable and secure seal. System designers can be more confident when choosing an engineered sealing solution.

Proper Metal-to-Metal Seal Installation & Damage Prevention

Modern transmission system design often results in very small port wall thicknesses, and most transmission components have little tolerance for deformation. These factors can be especially challenging for seals that require high installation forces – such as the stresses caused by ball bearing insertion, or the axial forces created by screw plug insertion. Both of which can result in cracks and other damage.

“Expander seals evenly distribute installation stresses,” said Tom Ryan, Head of Product Management at SFC KOENIG®. “The plug is inserted into the channel and then expanded, forming a tight seal with the base metal. Pull-style plugs eliminate axial forces and radial forces can be engineered based on the installation, making them ideal for thin-wall applications and softer base metals.”

Transmission ports can also be difficult to access, and housings can be large, which results in complex handling and positioning of the seal. Many expander seals uniquely support installation at difficult angles without the need for elaborate fixtures. Their design also allows for plugs to be easily set deep within the channel, which can eliminate dead space within the port where air pockets can occur.

Eliminating the Risk of Contamination

Different metal-to-metal seal types have different contamination sources, such as:

  • ball bearings – interference press-fit can result in metal debris

  • screw plugs – tapping threads can result in metal debris

  • cup plugs & screw plugs – often require sealant, which can be applied incorrectly or in excess, and contaminate the lubricating fluid or potentially block the port.

Expander seals require no additional machining processes or sealant, significantly reducing the risk of contamination.

Preventing Corrosion

Many transmission materials, such as cast iron, are not compatible with aluminum or other common seal metals. The engineer must consider the potential for corrosion between the seal and base, as well as between the seal and the fluid being sealed, especially after long lifecycles.

“Corrosion is an important factor in contamination and seal failure,” noted Ryan. “It is why SFC KOENIG offers a variety of material types, and why we have performed tests that demonstrate the effectiveness of our parts in corrosive environments. It is important to consider long-term corrosion factors when selecting the seal.”


When comparing the per-piece cost of sealing options, the less expensive options are screw plugs, cup plugs and ball bearings. Ryan noted that cost evaluation is more complex.

“When total costs are calculated, common seal types often turn out to be more expensive.” For example, our KOENIG EXPANDER® requires no additional machining processes and, with its one-piece design, requires no assembly, reducing error, scrap and labor costs. There is also significantly less risk of contamination, damage and seal failure. When total costs are calculated, expander plugs are often not only one of the most reliable, but also one of the most economical, metal-to-metal seal options available for drilled holes.”