For mechanics, there’s nothing like hearing that hydraulic fluid shooting all over your garage floor. It means you have used a fitting that is the wrong size and something is escaping at too high a pressure. The JIC thread chart is created to avoid this mishap. But too many folks dismiss it until they are wrestling with broken fittings and oily messes.
That means the confusing part is always where people think if two things look similar, they are going to be the same size; they’re not. The infographic also shows off the thirty-seven-degree flare angle as the shape needed with these types of fitting. That cone shape seals metal to metal against the seat of the mating piece; there’s no need for paste or tape to keep it in place. Tighten down your nut, the flare collapses around the seat and voila, tight joint. Use an SAE fitting with a forty-five-degree angle and you’ll crush the softer metal without actualy sealing it and immediately have a problem under load. It is a small difference but it can cost you big-time with pricy gear.
How to Use JIC Fittings Correctly
You won’t have to guess what size to order. Their dash numbered system eliminate any guessing. Sizes start with Dash two all the way through Dash thirty-two and they are laid out on the chart. Each number represent the outside diameter of the tube, plus or minus one-sixteenth of an inch. If you have a quarter-inch line, then you use a Dash four fitting. Half-inch line go into a Dash eight fitting. So as you move up, you go up by ones and it makes managing your inventory easier without having to take out set of calipers and measure a tube every time you get a tube out of that bucket. They simply stamped numbers on them for the tubing so you can see what dash size matches what and pull the right size. That may be a minor thing, but when you’re down there underneath a truck working in the dark it makes life a little bit easier.
The guide goes on to outline other choices like material which adds a whole new level to the decision process. Smaller hoses made with carbon steel has a high strength rating of up to 3,000 PSI, but the steel rusts when exposed to salt air. Stainless steel take care of the corrosion issue in chemical or marine applications at a higher price point. For fuel line applications, brass is soft enough that the threads won’t gall making it easier to assemble. Aluminum is used in racing builds due to its low weight, but it strips easily if you overtighten it.
One caution about aluminum, it strips if over-torqued, but it looks great in racing builds because it keeps them light. Note the torque values given in the reference chart. Alum yields much faster then steel. Go past the value by much and you’re looking at stripped fittings and wasted time.
It’s also a common myth among us in the shop that you need some sort of thread sealant on these connections; nope. A couple of wraps of teflon tape or a bit of pipe dope around a JIC fitting can interfere with your flare seats lining up correctly and cause leaks. These fittings use a cone shape to seal, not the threads themselves which just bring them into contact tight enough for the metal to bite and hold.
Standard threads are either UNF (straight) or UN (also straight), as opposed to the more familiar NPT (tapered). Straight threads does not damage the sealing surface while tapered ones do. Don’t overtighten: First, align the components by finger tightening the nut onto the hose end fitting. Then, use a wrench to rotate the nut only another one-sixth of a turn. That’s called “flats from finger tight.” By doing this you avoid cross threading the nut and you apply even tension on the seal without damaging its integrity. Tightening too much distorts the flare and creates stress points where vibration will cause cracks, this compromises the integrity of the connection.
How do you master JIC fittings? Well, that’s where the geometry comes into play. You could of respected it. It’s a 37 degree fitting angle; that takes care of all the sealing business. All you need to do is respect torque limits and choose the correct material. Follow the chart and let the metal-to-metal seal do its job, and the hissing will cease. That is the quiet operation you are looking for in a working system.
