Sealing the Deal: Everything You Need to Know About Thread Sealant for Hydraulic Fittings

Sealing the Deal: Everything You Need to Know About Thread Sealant for Hydraulic Fittings
When working with hydraulic systems, a tiny leak can lead to huge headaches—ranging from reduced efficiency and fluid loss to outright system failure. That's why the choice of thread sealant for your fittings is so critical! It's the unsung hero that ensures a reliable, leak-free connection.

This post will dive into why you need thread sealant, the different types available, and how to apply it correctly.

Why Use a Sealant on Hydraulic Threads?

You might think that simply tightening a fitting is enough, but most straight-threaded fittings (like ORB/SAE O-Ring Boss or JIC/AN 37° Flare) actually seal via an O-ring or a metal-to-metal contact on the face or cone, not the threads themselves. However, tapered pipe threads (like NPT/NPTF) are different.

Tapered Threads (NPT/NPTF): These threads are designed to create a metal-to-metal seal by wedging together. However, due to inherent imperfections and the helical path, there are always gaps and a leak path that a sealant must fill. A sealant is mandatory for NPT/NPTF fittings.

Straight Threads (e.g., ORB, JIC): Sealants are not typically required or recommended for the thread portion of these fittings, as they rely on a separate O-ring or flare mechanism to stop the leak. Using sealant here can sometimes prevent proper seating of the primary seal.

The Two Main Contenders: PTFE Tape vs. Liquid Sealant

For hydraulic applications, you generally have two reliable options for sealing NPT/NPTF threads:

1.PTFE Tape (Teflon Tape)

Pros: Easy to apply, inexpensive, inert to most hydraulic fluids.

Cons: Can shred or tear, introducing small pieces of solid material into the hydraulic system which can contaminate pumps or clog valves. It’s also easy to apply too much or too little.

Hydraulic Note: While commonly used, its potential for contamination makes it a less preferred choice for critical, high-pressure hydraulic circuits. Always leave the first 1-2 threads bare to minimize contamination risk.

2. Liquid Anaerobic Gasket/Thread Sealant (The Hydraulic Standard)

Pros: Excellent choice for high-pressure hydraulics. It's a liquid that cures (hardens) in the absence of air and in the presence of metal ions. It fills all gaps perfectly, creating a reliable, high-strength seal.

Cons: Requires a specific curing time and can be difficult to disassemble if a high-strength (permanent) grade is used.

Key Feature: The excess material that squeezes into the hydraulic line remains a liquid because it's exposed to air and won't cure. It gets flushed out harmlessly, virtually eliminating the risk of solid contamination.

Choosing the Right Type: Always select a sealant specifically rated for high-pressure, hydraulic applications and compatible with your system's fluid (e.g., mineral oil, synthetic). Look for medium-strength grades so you can disassemble the connection if necessary.

Best Practice Application Guide (For NPT/NPTF Threads)

Proper application is key to a leak-free system. Follow these steps when using a liquid anaerobic sealant:

Clean the Threads: Ensure both the male and female threads are free of oil, grease, and dirt. Use a clean solvent and allow it to dry completely. Clean threads are essential for the sealant to cure properly.

Apply Sealant: Apply a bead of liquid sealant around the circumference of the male pipe thread.

Mind the First Threads: Do not apply sealant to the first 1-2 threads at the very tip of the male fitting. This prevents the sealant from being pushed into the hydraulic system upon assembly.

Assemble: Thread the fitting in by hand until snug.

Torque: Use a wrench to tighten the fitting to the manufacturer's specified torque.

Cure: Allow the assembly to cure according to the sealant manufacturer's instructions before pressurizing the system. This often takes several hours.

Final Takeaway

For a durable, high-pressure, and reliable hydraulic system, the consensus is clear: Liquid Anaerobic Thread Sealant is the superior choice for sealing NPT/NPTF fittings. It minimizes contamination risk and provides a stronger, more consistent seal than PTFE tape.