SAE J518 Flanges: Reliable Connections for Hydraulic Systems

author: Qishine Industry

2023-09-12

One-Minute Overview

SAE J518 split flanges come in two codes: 61 for about 3000 psi and 62 for about 6000 psi.
They seal with an O-ring in a groove, not a cut gasket, so there is no gasket to crush or misalign.
Forged carbon steel is the default; stainless and alloy grades cover corrosive or low-temperature service.
Bolt torque, not pipe dope, makes the seal - follow the four-bolt sequence and the prescribed values.

SAE J518 Flanges at a Glance

SAE J518 (now aligned with ISO 6162) defines the four-bolt split flange and ring flange used to connect hydraulic pipe and tube. These flanges replace threaded fittings where vibration, pressure, and frequent teardown would loosen a joint. This guide covers the two flange codes, pressure ratings, material choices, how to size one, and the torque and welding steps that keep a hydraulic flange connection leak-tight.

What SAE J518 Specifies

The standard covers the flange halves, the O-ring groove, the bolt circle, and the socket for the tube or pipe. Two families matter in practice. Code 61 (split flange) suits working pressures near 3000 psi (210 bar). Code 62 (heavy split flange) reaches about 6000 psi (420 bar). A third, heavier ring-type flange is used where very high pressure or a rigid pipe interface is required. Dimensions and bolt sizes are fixed by nominal bore, so a flange from one compliant supplier fits another.

Code 61 vs Code 62 vs Ring Flange

Use this comparison to choose the flange family before you size the bore:

Feature	Code 61 (split)	Code 62 (heavy split)	Ring flange
Working pressure	~3000 psi / 210 bar	~6000 psi / 420 bar	Higher still (per size)
Seal type	O-ring in groove	O-ring in groove	O-ring / metal face
Typical bore	1/2 to 2 in	1/2 to 2 in	Per standard
Best use	General hydraulics	High-pressure, mobile	Rigid pipe, extreme duty

Material and Pressure Rating

Pick the material by the fluid and the environment, not just by price:

Carbon steel forgings (close to ASTM A105) are the workhorse for mineral and synthetic hydraulic oil.
Stainless (304 or 316) for washdown, marine, or food-adjacent lines where rust would seize bolts.
Confirm the pressure rating against the smallest component - the flange, the O-ring, and the bolt circle all set the limit.
For fluids above the O-ring's temperature range, move to a metal-faced seal or a different connection.

How to Select the Right Flange

Match the flange to the tube or pipe outside diameter and the pressure class, then pick the seal and material.

Read the line pressure and the peak (not the average) - size for the spike, which sets Code 61 vs Code 62.
Match bore to the tube OD; a loose fit invites fatigue cracking at the socket weld.
Choose the O-ring compound for the fluid and temperature (NBR for standard oil, FKM for heat, EPDM for water-based).
For a flat-face joint that needs a cut gasket, size the gasket to the same bore and pressure class.

Installation: Torque and Socket Weld

Prep the socket weld to the tube OD with the correct gap, tack and finish the weld, then fit the O-ring and the flange halves. Torque the four bolts in a cross pattern to the value in the flange table - typically a snug, even load rather than maximum force. Over-torque cracks the flange or splits the O-ring; under-torque weeps. Re-check after the first pressure cycle, because the O-ring and the joint settle.

SAE J518 vs Threaded and ORFS Fittings

Two comparisons come up most often on a hydraulic skid:

Against NPT threads, the split flange wins on vibration and teardown - threads gall and leak. Against ORFS (O-ring face seal) adapters, the flange carries more load on large bore and is easier to service on a manifold. Use the flange where the pipe is large or the joint is opened often; use ORFS for smaller tube runs.

FAQ

What does SAE J518 cover?

Four-bolt split and ring flanges for hydraulic pipe and tube, with the O-ring groove and bolt-circle dimensions fixed by nominal bore.

What is the pressure rating of a Code 61 vs Code 62 flange?

Code 61 is rated near 3000 psi; Code 62 near 6000 psi, both by nominal bore. Always confirm against the full flange table.

Does an SAE J518 flange need a gasket?

No. It seals with an O-ring in a groove. Only flat-face flange variants that use a separate gasket need one.

What torque should I use?

The value in the flange's specification table, applied in a cross pattern. Do not over-torque - the seal depends on even load.

Which O-ring compound for hydraulic oil?

NBR for mineral oil, FKM for high heat, EPDM for water-glycol. Match the compound to the fluid and the temperature.

Can I mix Code 61 and Code 62 flanges?

No - the bolt circle and O-ring groove differ. Pair like with like, or the joint will not seal.

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