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Hydraulic Breaker Seal Types Explained: Complete Guide + Installation Tips

Choosing the right hydraulic breaker seal kit starts with understanding the different seals inside your rock breaker. Each seal has a specific job and knowing them helps you install parts correctly and avoid costly mistakes.

At Seal Kit City, we supply complete aftermarket seal kits and diaphragms for popular models like Atlas Copco/Epiroc SB452, Rammer BR4099, CAT H120, Kent/Furukawa KF9, and more.

Why Seal Types Matter

Worn or wrong seals cause leaks, loss of power, overheating, and expensive downtime. Replacing the full seal kit (including the accumulator diaphragm) is the best way to restore performance.

1. Piston Seals (Main Power Seals)

Function: Prevent high-pressure oil from leaking past the piston during the hammering cycle. Common Materials: HNBR or Polyurethane (PU) Installation Tips:

  • Lubricate generously with clean hydraulic oil before installing.
  • Use a seal installation tool or plastic sleeve to avoid cutting or rolling the seal lips.
  • Make sure the seal is seated squarely in the piston groove.
  • Check for any scoring on the cylinder wall before reassembly.

2. Rod Seals / Guide Seals

Function: Seal the piston rod as it moves in and out of the front head. Installation Tips:

  • Install with the open side of the U-Cup facing the pressure (hydraulic oil).
  • Use a rod seal installer tool to prevent damage.
  • Avoid twisting the seal during installation — this is a very common mistake.
  • Lightly grease the rod before sliding it back through the new seals.

3. Buffer Seals (Pressure Backup)

Function: Absorb sudden pressure spikes and protect the main rod seal. Installation Tips:

  • Usually installed right behind the primary rod seal.
  • Ensure they sit flat with no gaps.
  • Replace every time you do a full seal kit — they wear faster than most people realize.

4. Wiper / Dust Seals (Scraper Seals)

Function: Wipe dirt, dust, and rock debris off the piston rod. Installation Tips:

  • The sharp lip faces outward (toward the environment).
  • Clean the rod thoroughly before reassembly — any dirt left behind will destroy the new wiper seal quickly.
  • These are inexpensive but critical for long-term reliability.

5. Accumulator Diaphragm

Function: Separates nitrogen gas from hydraulic oil to maintain consistent hitting energy. Note: This is a flexible membrane, not a traditional seal. Installation Tips:

  • Pay close attention to orientation (convex side usually faces the gas chamber).
  • Torque the accumulator cover bolts in a star pattern to manufacturer specs.
  • Pre-charge with nitrogen after installation (typically 100–150 psi — check your model).

6. O-Rings & Backup Rings

Function: Static seals used in ports, covers, and end caps. Installation Tips:

  • Always use new O-rings — never reuse old ones.
  • Install backup rings on the low-pressure side of the O-ring.
  • Lightly lubricate before installing.

Seal Material Quick Reference

Material Heat Resistance Abrasion Resistance Best For
NBR Good Good Light to medium duty
HNBR Excellent Very Good Most rock breakers (recommended)
Polyurethane Good Excellent High-wear, dusty conditions
Viton Outstanding Good Extreme heat or chemicals


Pro Tips for Successful Seal Kit Installation

  • Always install a complete seal kit — mixing old and new parts causes early failure.
  • Work in a clean area — contamination is the #1 enemy of new seals.
  • Torque all bolts to spec. Under- or over-torquing causes leaks.
  • After installation, run the breaker at low pressure for the first 10–15 minutes and check for leaks.

Lubrication Tips – Using Lithium Grease

When installing seals:

  • Best Option: Clean hydraulic oil (same as your machine uses).
  • Good Alternative: White lithium grease or general-purpose lithium grease.

How to use lithium grease correctly:

  • Apply a very thin layer on the outer surface and sealing lips only.
  • Do not pack the grooves full of grease.
  • Wipe away any excess before reassembly to prevent contamination.
  • Many techs use a 50/50 mix of hydraulic oil and lithium grease for best results.

Important: Never use silicone grease, heavy chassis grease, or molybdenum disulfide (MoS2) grease.

Need the right seal kit for your breaker? Call us at 844-2-SEALKIT and give us your make, model, or serial number. We'll get you the exact kit fast.