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Sealing Screw or Sealant?

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When choosing sealing options for equipment, engineers sometimes ask if adhesives applied to fasteners work as well as sealing screws and bolts. The short answer is no. Let’s delve into why.

What’s the difference?

A sealant is a material used to fill and seal gaps, joints, or seams to prevent passage of liquids or gases. It is often applied to surfaces or between plates in a liquid or paste form and then cures or hardens over time to create a protective barrier. 

A seal screw is machined with a groove under the head of the screw that fits a rubber O-ring. Once the seal screw is torqued, the rubber O-ring expands to create an impenetrable 360⁰ seal. This seal prevents contaminants, like oils, liquids, gas, moisture, and precipitation from seeping into equipment and damaging it or out and potentially harming the environment.

If you are designing military and aerospace equipment, the undercut O-ring groove system is best. Unlike sealants, the undercut O-ring groove system must meet strict military protocols and is pressure-tested to 1,500 psi. 

Advantages of Undercut O-Ring Groove:

  1. Protects the O-ring from being damaged.
  2. Eliminates over-compression when squeezed.
  3. Allows O-ring to move freely when screwed.
  4. Prolongs reusability of the screw.
  5. Enables screw to withstand a high-pressure environment.
  6. Prolongs equipment life; minimizes malfunction and costly repairs.
  7. Meets strict Mil Spec standards.

Disadvantages of Sealant:

  1. Have difficulty withstanding temperatures 250⁰ and above; sealant becomes gummy or sticky.
  2. Bonding separates when exposed to certain chemicals, including oils.
  3. Degrades over time, including cracking, blisters, and voids.
  4. Risks installation damage because of lack of groove protection.
  5. Becomes flat without a groove and prone to breakage.
  6. Does not meet strict Mil Spec protocols.

Have questions?

ZAGO sealing fastener specialists are here to help you choose the best sealing fastener solution for your application. Please contact our tech specialists at

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