A Flair for Flaring

How to Make Tubing Flares the Right Way

By Jeff Smith – Photography by the Author

There are certain skills that every car guy worth his 9/16-inch end wrench should know how to perform. Drilling holes in the proper place, cutting threads, and general fabrication skills are not difficult. High schools used to teach these lessons in metal shop, but not anymore. So this new generation of car guys will have to learn it on their own.

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The skill we will concentrate on today is creating a leak-free tubing flare. There are actually several different angles that are used as well as metric bubble flares used on import cars. We’ll maintain our focus on the two most popular standards, the 45-degree flare and the AN 37-degree flare. These will require different tools, and we’ll cover that as well.

01 professional-level hydraulic flaring tool
There are many different flaring tools available, from the simplest to the more complex and some professional-level hydraulic tools that are really nice.

Making tubing flares requires a specific tool, so you’ll have to invest in a good tubing flare set. Let’s assume that you will use this tool forever. This precludes that $10 package you will find at the auto parts store hanging on the pin in the tool section. We did the same thing; bought a cheap flaring tool only to discover how much nicer the flares worked with a quality tool. Invest in your automotive future with good tools.

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Most tools will accommodate several tubing sizes–the most common being 3/16-, ¼-, 5/16-, 3/8-, and ½-inch tubing sizes. The less expensive tools will also limit your flaring to mild steel tubing as the tool may not be strong enough to work with stainless steel tubing. As a point of reference, it’s best to make sure that if you will be using stainless tubing you only buy tubing that has been annealed. This is a heating process that softens the somewhat brittle stainless, making it easier to flare and bend. This is important because not all stainless steel tubing is annealed. If it doesn’t say it’s annealed, it probably isn’t.

02 flaring tool tips first step cutting the tubing
The first step is cutting the tubing. Standard tubing cutters lead a thin burr on the inside of the tubing that must be removed to achieve a good flare.

We will run through the process of making a 45-degree flare on a 5/16-inch length of tubing. This is commonly used on automatic transmission cooler lines and it’s not unusual to have to make your own hard lines for the installation of a new transmission cooler.

We’ll run through the process in the step-by-step photos along with including some details that will help you create the best flare possible. You will likely spend more time preparing the tubing for a flare than making the flare itself. If the tubing is not cut straight or the outside diameter is not consistent or has sharp edges, the probability of making a successful flare is low.

03 flaring tool tips Cutting tubing always leaves sharp edge on the inside of tubing
Cutting tubing always leaves a sharp edge on the inside diameter of the tubing.

The most common way to shorten tubing is with a tubing cutter. Tubing can be cut with a hacksaw or cutoff wheel but will likely require more time to ensure the end of the tubing is cut perpendicular. A tubing cutter generally makes a very straight cut but will leave a small burr or sharp edge around the inside circumference of the tubing. This must be removed. Most tubing cutters include a small wedge-type tool that can be used to remove these burrs, but we’ve learned these can often be ineffective.

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We found a handheld tubing reamer at a local Grainger store that works well for both mild steel and stainless tubing. There are also large round plastic deburring tools that will both clean up the inside and outside circumferences of the tubing, which is helpful.

Once the tubing has been deburred, we’ve found that polishing the end of the tubing with a convolute wheel on a bench grinder also improves the chances of making a good seal. This allows you to place a small chamfer on the outside of the tubing.

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04 flaring tool tips this reamer at Grainger that works very well at removing the burr
We found this reamer at Grainger that works very well at removing the burr rather than relying on that wedge-shaped device on the tubing cutter.

Before you start the flaring process, make sure the fitting is in place and facing in the proper direction. We’ve all made the mistake of completing a great flare only to discover that we forgot to slide the fitting in place.

The next step is placing the tubing in the tool. This is an important step because the height of the tubing establishes the amount of material that is bent inward. Set the height of the tubing using the step on the flaring die to match the height and tighten the clamp around the tubing starting with the clamp closest to the tubing. Both clamps need to be tight to prevent the tubing from being pushed out of the holder.

05 flaring tool tips polishe the ends of tubing after cutting and deburring to create smooth surface for shaping
A professional car builder once mentioned that he polishes the ends of the tubing after cutting and deburring to create a smooth surface for shaping.

We like to place a small drop of oil on the die right before we put it into the tubing. This helps make the die last longer and offers lubrication to make the process easier to accomplish. The adapter bends the tubing inward slightly before it bottoms out. Once it bottoms, loosen the load, remove the die from the tubing, and then place the cone of the anvil directly into the tubing and complete the flaring process. This creates a double flare that will then seal properly.

Summit Racing also offers an interesting professional flaring tool that uses mandrel and steel dies with a handle with sufficient leverage that makes it quite easy to form 45-degree double flares. This is still a two-step process like the simpler flaring tools but accomplishes the process much more quickly. Plus, there are dies available separately for 37-degree AN flares.

06 flaring tool tips Matco handheld flaring tool
We are using our Matco handheld flaring tool for this procedure but the process is the same for all handheld flaring tools. Use the step on the die to set the height of the tubing. Then place the adapter inside the tubing. If the die does not fit inside the tubing, it’s likely the inside has not been properly chamfered. This enlarged photo reveals a slight deformity in the tubing that we didn’t see during the flaring process.

If you are really into tubing work or are planning on doing this professionally, there are several companies out there selling a hydraulic flaring tool that uses the same dies as the Summit tool we just described but uses a hydraulic pump to do the work for you. We’ve used the Classic Tube tool and it’s both quick and easy. As with the Summit tool, this requires separate mandrels for 37-degree AN flares versus the normal 45-degree flares along with metric and even the GM quick-release–style tubing flares.

Up until now we’ve focused on the standard 45-degree double flare but in the hot rod world, much of the plumbing is accomplished with 37-degree AN fittings. These are finished as a single-flare operation, so from that standpoint, the AN process is slightly simpler. But this also requires a separate or specific tool to make this flare. The 45- and 37-degree flares are not interchangeable and while in an emergency you might be able to make a combination seal, it will likely damage one or both of the sealing surfaces.

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07 flaring tool tips Tighten anvil until the die bottoms on the base of tool
Tighten the anvil until the die bottoms on the base of the tool.

Many years ago we bought one of the Rigid AN flaring tools and it has since become a go-to tool because it is simple and easy to use. When using the annealed stainless tubing for AN flares, ensuring that the inside diameter of the tubing is properly radiused with no sharp edges is a critical part of the flaring procedure. AN female fittings for tubing use a tubing nut and sleeve placed over the tubing before making the flare. Make sure the flared portion of the tube nut faces the flare. Work slowly here and you will be rewarded with a professional-looking fitting that will seal every time you make the connection.

08 flaring tool tips place in vise to make making the flare easier than holding it in your hand
The base of the flaring tool is long enough to place in a vise to make making the flare easier than holding it in your hand. That’s what we’ve done here for the last step of removing the die and tightening the cone into the tubing to complete the flare.

As you can see, there’s much more to the world of tubing and making automotive fluid connections. It still comes down to using quality tools and working carefully to ensure all the steps are completed properly. Nobody likes spending time making a flare only to discover it leaks when tightened. On the other hand, a properly produced flared tubing fitting could last almost the life of the car.

09 flaring tool tips A successful completed flare will look like this
A successful completed flare will look like this. If the flare appears eccentric or out of shape, it will need to be redone because it will not seal properly.
10 flaring tool tips brass fittings like these can be over-tightened, which deforms the soft, coned sealing surface
It’s been our experience with carburetor fittings and other female brass fittings that these can be over-tightened, which deforms the soft, coned sealing surface. Once deformed, the fitting will likely leak even with the best 45-degree flare on the tubing.
11 flaring tool tips Summit offers this slick upgrade to the double flare tool
Summit offers this slick upgrade to the double flare tool that uses dies and mandrels with a 12-inch handle that offers leverage with less effort. This is still a two-step process to make a double flare but does so in less time.
12 flaring tool tips This Rigid eccentric tool is great for making single flare
This Rigid eccentric tool is great for making single flare, 37-degree AN fittings.
13 flaring tool tips AN tubing flares will require both tubing nut and tubing sleeve to complete fixture
To complete the connection AN tubing flares will require both a tubing nut and a tubing sleeve to complete the fixture.

Parts List

Description PN Source
Klein tubing cutter 88904 Summit Racing
Tubing cutter 91089526 Speedway Motors
Mini tubing cutter 900500 Summit Racing
Tubing Reamer 3CYP2 Grainger
Summit Professional Flaring tool 900314 Summit Racing
Summit 45-degree flaring tool 9003310 Summit Racing
Rigid 37-degree flaring tool 91089524 Speedway Motors
Matco double flaring master kit DFS260B Matco Tools
Classic Tube hyd. flaring tool CST-7207 Summit Racing


Grainger Industrial Supply
(800) 472-4637

Matco Tools
(888) 696-2826

Speedway Motors
(855) 313-9173

Summit Racing
(800) 230-3030

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