Rajan Wilcox
Flaring is the last step in building a custom line for a fluid transfer project. To double flare a 3/8 fuel line, you need to cut the raw hard line to length, bend the line to match your template, and flare the ends for installation. The 45-degree inverted flare is the most common style of flare used in automotive hard lines like brake, fuel, and transmission fluid transfer. The 37-degree double flare tools are expensive and rare.
How to Double Flare 3/8 Fuel Line
| Characteristics | Values |
|---|---|
| Cutting the line | Use a tubing cutter to cut the raw hard line to length |
| Chamfering | Use the tubing cutter's built-in chamfer blade to create a clean cut |
| Bending | Use a tube bender to bend the line to match your template |
| Flaring | Use a 37-degree or 45-degree flare tool to flare the ends of the line |
| Fitting | Install the correct size fitting for the master cylinder or other hardware |
| Single vs Double Flare | A single flare may be sufficient for stock fuel lines, but a double flare provides extra wall thickness |
| SAE Standard | The 45-degree inverted flare is the SAE standard and is created in a two-step process |
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What You'll Learn
Cutting the line to length with a tubing cutter
When using a tubing cutter, it is important to measure the length of the line accurately before making the cut. This will ensure that your line is the correct length for your specific application. You can use a tape measure or ruler to measure the length of the line before cutting. It is also important to ensure that the tubing cutter you are using is the correct size for the tubing you are cutting. Tubing cutters typically have adjustable blades that can be positioned to fit the size of the tubing.
Once you have measured the length of the line and adjusted the tubing cutter to the correct size, you can begin cutting. Position the tubing cutter around the tube, ensuring that the blade is in contact with the tube at the desired cutting point. Then, simply rotate the tubing cutter around the tube, applying firm and consistent pressure. As you rotate the cutter, the blade will gradually cut through the tube.
It is important to take your time and cut slowly and steadily to ensure a straight and accurate cut. Rushing the process can lead to an uneven or jagged cut, which can cause problems when flaring the tube ends. Once you have made a full rotation with the tubing cutter, you can gradually tighten the cutter and continue rotating until you have cut through the tube completely. Finally, use a deburring tool or sandpaper to remove any burrs or sharp edges left over from the cutting process.
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Bending the line with a tube bender
Once you have cut your fuel line to the correct length, you will need to bend it to match your template. This is done with a tube bender. There are several types of tube benders available, each with its own advantages and disadvantages.
Simple benders
Simple benders are single-piece tools that use a hook-type latch to lock the tubing in place. One hand holds the line and tool together, while the other hand bends the line. Simple benders are more likely to kink the line, especially with harder lines.
Spring-type benders
Spring-type benders are very basic benders meant mainly for copper and aluminium tubing. There are two types: internal and external. Both prevent the tube from collapsing and kinking while hand bending.
Lever-action benders
Lever-action benders use mechanical advantage to make bending lines easier. The tubing can still rotate, but it is easier to manage with a lever-action bender. Lever-type benders typically have multiple diameter positions and markings for varying degrees of bend. Most of these tools are designed for specific radius bends, and there are tools for tight bends and large bends. You can manipulate this to a degree by repositioning the line, making a larger radius. However, you can’t make a smaller radius than the tool you have.
When using a bending tool, it is important to get a quality tool that won’t kink the line.
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Using a 37-degree flare tool or a 45-degree flare tool
On the other hand, the 45-degree flare is a two-step process that results in a double-walled flare seat. This type of flare is an SAE (Society of Automotive Engineers) standard and is the most common style used in automotive hard lines, including brake, fuel, and transmission fluid transfer lines. It features external or male threads on the fitting, and due to the extra wall thickness, it does not require a tube sleeve or any other type of flare seat support.
Both types of flares will effectively seal your fuel line, and the choice between the two ultimately comes down to personal preference and experience. If you plan to reuse the tool for other projects, consider that the 45-degree flare is more common for automotive applications, especially when working with stock components.
When using either the 37-degree or 45-degree flare tool, follow these general steps:
- Cut the raw hard line to length using a tubing cutter to ensure a clean cut.
- Bend the line to match your template using a tube bender.
- Insert the line into the correctly-sized opening in the clamping bar of your chosen flare tool.
- Set the line depth according to the tool's instructions and secure the line.
- Use the tool's forcing cone to create the desired flare. For a double flare, you will need to use the forcing cone double flaring adapter for the first step of the process.
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Using a forcing cone double flaring adapter for a double flare
To create a double flare using a forcing cone double flaring adapter, you will need to follow these steps:
First, you need to cut the raw hard line to length. For this, you can use a tubing cutter, which will provide a clean cut. Using a hacksaw or cutting wheel is not recommended as it can create filings or debris that can damage sensitive fluid systems.
Next, you will need to bend the line to match your template. This can be done using a tube bender or bending tools/pliers specifically designed for the task. It is important to ensure that the line is bent correctly before moving on to the next step.
Once your line is cut and bent to the correct specifications, you can begin the flaring process. For a double flare, you will need to use a forcing cone double flaring adapter with your flare tool. The specific adapter and tool you need will depend on the material of your fuel line. Common materials include copper, aluminum, soft steel, and brass. Ensure that you have the correct size adapter for your line; standard sizes include 3/16-inch, 1/4-inch, 5/16-inch, 3/8-inch, and 1/2-inch.
When you are ready to begin flaring, insert the hard line into the correctly-sized opening in the clamping bar of your tool. Set the line depth according to the tool's instructions, and then secure the line. The forcing cone double flaring adapter will be used to create the first step of your double flare. This process will vary depending on the specific tool and adapter you are using, so be sure to refer to the manufacturer's instructions.
Finally, it is important to note that double flaring requires additional time and specialised tooling. It may be more cost-effective to simply replace the brake lines instead of investing in the tooling required for double flaring. However, double flares are less likely to leak and can handle more fluid pressure and vibration due to the double-walled flare seat.
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Using a single flare for stock fuel lines
Using a single flare on stock fuel lines is a straightforward process that can be done with the right tools and some practice. Here's a step-by-step guide on how to do it:
First, you need to cut the fuel line to the desired length. It is recommended to use a tubing cutter for this step, as it creates a clean cut that can be easily chamfered with the tool's built-in chamfer blade. Using a hacksaw or cutting wheel might create filings or debris that can damage sensitive fuel system components.
Once you have your line cut to length, you need to bend it to match your template. For this step, you will need a tube bender. There are several bending tools available on the market, including dedicated brake line benders and bending tools/pliers for other line sizes. It is important to bend the line accurately to ensure proper installation.
After you have bent the line to match your template, you can move on to flaring the ends. For a single flare, you will need a 37-degree flare tool. This tool can be found at various price points to fit your budget. The 37-degree flare is a single flare process that requires a tube sleeve to support the backside of the flare.
When using the 37-degree flare tool, apply a drop of oil to the tool for a perfect flare. This will help you achieve a clean and precise flare every time. It is important to note that the line should have a straight enough section near the end to accommodate the flare.
For fuel lines, it is recommended to use compression fittings, especially with mild steel lines. Stainless steel lines can get away with using a single flare, but it is not recommended to double flare stainless steel as it is harder and more brittle than stock lines.
By following these steps and practicing on some scrap tubing, you can confidently create single flares for your stock fuel lines, ensuring a secure and leak-free connection.
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Frequently asked questions
The first step in preparing a fuel line for flaring is cutting the line to length. It is recommended to use a tubing cutter to create a clean cut.
The next step is bending the line to match your template. A tube bender can be used for this purpose.
The 45-degree inverted flare is the SAE (Society of Automotive Engineers) standard for fuel lines. It creates a double-walled flare seat.
A brake line flare tool is required to create a 45-degree flare, which is the most common style for fuel lines. A 37-degree flare tool can also be used, but it is less common and more expensive.
