How to Use a Flaring Tool? | The Proper Technique

Using a flaring tool requires a square cut, a clean deburr, the nut installed first, and a properly clamped tube before forming the flare.

Getting a leak-free seal on a brake line or a copper pipe comes down to preparation. Most failed flares aren’t the tool’s fault—they’re the result of a rough cut, a missed burr, or the flare nut sitting in the wrong spot. Here’s the step order that works, plus the mistakes that ruin the joint.

Step 1: Cut And Deburr The Tubing Cleanly

The flare can only be as good as the end of the tube it’s formed on. Cut the tubing with a tubing cutter, not a hacksaw, and work slowly so the wheel leaves a square, even edge. After the cut, remove the burr from the inside edge with the tool’s deburring blade—and chamfer the outside edge too, since a sharp outer lip can prevent the flare from seating correctly.

This step is worth the extra minute. An uneven edge or leftover burr can create a weak point that cracks when the cone presses into the metal.

Step 2: Slide On The Nut, Then Clamp The Tube

Install the flare nut before you make the flare. Once the flare is formed, the nut cannot be slid over it. With the threaded end facing away from the tube end, slide the nut on and out of the way while you clamp the tubing into the tool’s bar or die block.

Tube protrusion matters more than anything else at this stage. Some tools call for the tube to sit flush with the block; others want a small protrusion roughly equal to the adapter’s shoulder or the nut’s thickness. Check the gauge or marking on your specific tool instead of guessing—too little tube and the flare is undersized, too much and it folds over itself. Tighten the clamp securely so the tubing cannot slip when the cone forces its way in.

Step 3: Form The Flare With The Correct Adapter

Select the die and adapter that match the tubing size you’re working with. For a single flare, simply seat the cone over the tube and advance it until the flare is fully formed. For a double flare—which is what most automotive brake lines require—the process has two stages: first, advance the cone until the adapter bottoms out to create the inner fold, then remove the adapter and advance the cone again to flatten the finished flare.

Stop when you feel firm resistance. Over-torquing the tool risks splitting or wrinkling the flare. The official Eastwood manual for its brake tube flaring tool emphasizes inspecting the finished flare for concentricity and cracks before installation on a vehicle.

If you’re in the market for a reliable tool rather than working with a borrowed one, our tested roundup of the best flaring tools covers which kits handle brake lines versus soft copper tubing.

Flaring Mistakes That Ruin The Joint

Most failed flares trace back to a handful of errors, and they’re all avoidable:

• Skipping the deburr. Burrs can crack the flare as it forms and prevent proper seating.
• Wrong protrusion. Too little tubing creates a thin, weak flare; too much makes a wrinkled, oversized mess.
• Loose clamping. If the tube slips through the bar during forming, the flare is scrap.
• Over-tightening the tool. Extra torque doesn’t mean a better flare—it means a split one.
• Using the wrong die size or flare type. Single and double flares are not interchangeable across all fittings and applications.

A light lubricant on the cone can improve the finish and reduce friction, but that’s tool-specific advice—check your manual before applying anything to the working surfaces.