37deg Flared Tube (JIC)

Owned by Michael Zhou
Dec 20, 2022
4 min read

JIC (Joint Industrial Council) is a standard of end connection that specifies the dimension and form of a 37 deg cone and thread for fitting ends.

A typical JIC tube connection

Tubing can be connected to these fittings by flaring (expanding the end of the tube into a conical shape) the tube and and then tightening the connection against the cone of the fitting by using a sleeve and a nut. The nut grips the male threads on the fitting and pushes on the sleeve. The sleeve in turn pushes against the flare of the tube, which seals the inside of the flared tube against the outer surface of the cone of the fitting (also called the nose).

JIC fittings are an industrial standard, which means that they are much cheaper and more readily available compared to Swagelok fittings. (Thanks Regional Hose!)

We typically use stainless steel fittings for material compatibility reasons. Stainless offers the best chemical resistance at a decent price.

JIC vs AN

There is frequent confusion over the difference between JIC and AN ends. They are identical except for the thread profile. AN fittings are usually manufactured to a tighter tolerance, for use in aerospace, compared to JIC fittings for use in industries with less strict requirements.

While the thread size and pitch are the same for both JIC and AN, the thread profile for an AN end is 3A UNJF compared to 2A UNF. A J-series thread has a controlled root radius on the external thread, which helps with preventing stress concentrations and crack formation in a high stress/vibration environment.

However, due to these differences in thread form, there is the possibility of interference if AN and JIC fittings and nuts are used together, which can cause binding, galling, and leaks.

Getting a Reliable Connection

Getting a good seal with flared fittings requires:
Good quality fittings

  • AN fittings are ideal, while quality of JIC fittings can vary enormously since it's a looser industrial spec - I think the stuff we've gotten from Regional hose is of pretty good quality

Good quality flaring tool

  • Adam Trumpour has a couple Imperial flaring tools as well, and says they work well

Good flaring process - TODO: write a procedure

  • Tube end preparation: saw, clean up with a file or mill, carefully deburr the ID and OD.

  • Heavily lubricate the rollers on the flaring tool, follow the instructions, do a good burnishing pass

  • Make sure OD of flare is within spec

Good installation practice

  • Stainless fittings are prone to getting galled at the nose (male cone) when tightening, especially if it's machined fitting against machined fitting. It just be like that

  • Strongly recommended using conical seals (crush gaskets) over the nose of the fittings to prevent galling and to protect the cone/extend the life of a worn cone, usually made of copper, aluminum, or nickel (example: https://www.pegasusautoracing.com/productselection.asp?Product=3202 ) - requires slightly higher torque when tightening than without a conical seal

  • Proper prep of the joint is important when assembling. Use a little krytox on the bearing surfaces, including the back of the sleeve, male thread, and front of the sleeve where it bears against the tube

  • Large AN or JIC fittings require a lot of torque, one of the big disadvantages of them

  • Joint should be able to be tightened finger tight initially, NEVER force a slightly misaligned connection

  • Use a good quality torque wrench and set of crowsfoot, follow torque spec for the fitting size and material

  • Torque to bottom of spec, then increase torque if there is a problem

  • There is usually a large radius where the flare transitions into the tube, can cause sleeve binding. Fix by tightening to a fitting to fully form the flare into shape - sleeve should be able to slide and rotate freely when pushed up against the back of the flare after

  • get a good, smooth male fitting with a conical seal, lube the crap out of it

  • lube the back side of the flare on the tube (the contact surfaces between the sleeve and the tube / flare)

  • assemble and torque to the top end of the torque spec, or just slightly above it. 

  • loosen the nut, rotate the tube about 120 degrees, and re-torque. Then do that one more time.

* Regional Hose JIC fittings are about 2-4x cheaper than the corresponding Swagelok fitting, and typically have a 3 day lead time as of Fall 2022. We should try to use these for high pressure plumbing going forward once we have a good procedure to make and assemble flared tube ends. - Michael