5/8-18 RH Inert Gas Fitting: Specs, Standards, and What to Know
What Is a 5/8-18 RH Inert Gas Fitting?
A 5/8-18 RH (right-hand) inert gas fitting is the standard outlet connection found on virtually every MIG and TIG welding regulator made. It’s a brass threaded coupling designed to connect gas regulators, hoses, flowmeters, and torches in arc-welding applications. If you’ve ever looked at the outlet side of a welding regulator, this is what you’re looking at.
Breaking Down the Designation
The full name—5/8-18 UNF RH—tells you several things at a glance:
- 5/8″ refers to the diameter of the thread, measured in inches
- 18 is the thread pitch, meaning 18 threads per inch (UNF = Unified National Fine)
- RH stands for right-hand thread, the standard direction you turn clockwise to tighten
This threading standard is set by the CGA (Compressed Gas Association) under specification CGA-032 for inert gases. The CGA designation distinguishes inert gas fittings from acetylene (which use left-hand threads as a safety measure) and oxygen fittings.
Specifications and Pressure Ratings
Standard 5/8-18 RH fittings are rated for a working pressure of 200 PSI and a burst pressure of 800 PSI. This is more than adequate for typical shielding gas delivery, which runs around 15–25 PSI for MIG welding and 10–20 PSI for TIG. The fitting itself is manufactured from solid brass for durability and corrosion resistance.
Which Gases Work with This Fitting?
Because it’s designed for inert gas applications, this fitting safely connects to:
- Argon (pure, most common for TIG and MIG)
- Argon/CO2 blends (standard for MIG)
- Helium (for specialized TIG and brazing)
- Nitrogen (less common but used in some processes)
The inert gas designation means these gases don’t react with the brass or the materials inside the regulator, making this the right choice for arc welding.
Why Thread Direction Matters
The right-hand thread is not arbitrary. In the welding world, acetylene hoses use left-hand threads (reverse-threaded), and this difference exists for safety. By making the threads different, you physically cannot connect an acetylene hose to an inert gas regulator outlet, or vice versa. Mixing up connections could lead to incompatible gases reaching your torch, which creates hazards. The notches or grooves sometimes visible on a fitting’s hex nut are a visual indicator of left-hand threads, so a quick inspection can confirm which type you have.
Common Configurations and Adapters
The 5/8-18 RH outlet on your regulator can connect to various adapter configurations depending on what’s on the other end. Common setups include:
- 5/8-18 RH female (on regulator) × 1/4″ NPT male (to hose or flowmeter)
- 5/8-18 RH female × 1/4″ male pipe thread for direct connection to hoses
- Straight couplings (5/8-18 RH × 5/8-18 RH) for joining hoses end-to-end
Replacement hoses typically come pre-fitted with 5/8-18 RH ends, so you can usually just unscrew the old one and thread on the new one without additional adapters.
Installation and Maintenance
When installing or replacing a hose or adapter, hand-tighten first, then use a wrench to snug it firmly—do not over-tighten. Brass is soft and can strip if you apply too much force. Check that the connection is gas-tight by applying soapy water; bubbles indicate a leak. Most fittings last for years if kept clean and dry. If you notice corrosion or difficulty threading, it may be time to replace the fitting.