Flare nuts is required for pressurized lines like fuel rails and brake hoses. These nut should be proper size. They sit in middle of the line joint. If they are not the right size, the nut won’t fit into the joint or it will leak under pressure.
Tube diameter, thread pitch, pressure rating, and wrench size is all available from the chart. So no guessing when working on the bench. As you can see in the diagram above, compression fittings cannot be substituted for flare nuts.
Why Use Flare Nuts
Flare nuts slides onto the tube, and then the end is shaped to press the flared conical section against a matching seat. This metal to metal connection can withstand more vibration and pressure different than one where a ferrule bites into the tube wall. The chart indicates that the pressure differential are substantial. Typical compression fittings max out much earlier while 37 degree AN fittings remains snug at thousands of pounds per square inch. For these reasons brake and fuel line nearly always employ flares.
There is two popular flare angles. The forty-five degree angle is typical of domestic fuel and brake lines in cars, while the thirty-seven degree (AN or JIC) angle are found on heavy equipment and aircraft. They may appear similar but they won’t mate together and create a sealed cone connection. Each row indicates its angle so that you can check your specification prior to purchasing anything.
Also shown is a note about double flared tubes where the tube wall are folded back onto itself, adding additional strength against cracking for most OEM brake line. The Metric sizes work the same way but have their own callout of thread size. You won’t need to do any awkward conversion for your Asian or European vehicle… There’s a table in the chart.
Again, whatever size wrench is listed applies to the wrench against the hex on the nut; not the body behind it on the fitting. Rounding off the corners with the incorrect wrench will make further service more difficultly. Down below are the torque values, as over-tightening wrecks a new flare fast. The advice here is simple. Hand-tighten until snug; then use your wrench and turn it one flat. Because steel nuts can handles higher torque than aluminum nuts, that’s how the chart divides those two columns.
Use a second wrench on the fitting body so you don’t twist up the line when you’re turning the nut. So what does it mean to the end user? In practice, there is no more guessing when looking at used fittings or in the parts aisle. First, match the tube OD, then match the thread and flare angle. Next look at the torque column and grab the wrench. By doing so, you would of known the fitting will hold pressure the first time and the tenth time.
