I am installing some new fuel line and had to cut off the double flare that came on it in order to shorten the line a bit. I've been trying to put a new double flare on the end to no avail. No matter what I do, the tubing backs out of the flaring tool when trying to do the first step of the double flare process with the insert you put on the tube. I've tried two different flaring tools (one of which I broke the wing nut off the end trying to tighten it). I tried clamping both of them in a vice, as well as adding C-clamps to add extra pressure and the tubing still pushes out. The tubing is regular steel, not stainless. Anyone have any tips as to how to keep the tubing from backing out of the tool? Also, this is the end of the tube where it ends at the gas tank and you connect the short piece of fuel hose to the sending unit. Since it is not under pressure, does it even need to be flared?

Hit the flair part on the end with hammer, the jolt makes it bend, might have to do a few times. Also make sure no grease on pipe.
You are using the insert I hope.

From my experience, the major difference between quality flaring tools and the cheaper ones is the way they grip the tubing. Quality tools like the Mastercool grip the tubing better, they don't slip.

Agree totally and got fed up with doing a flare multiple times to get it right so I went to Eastwood's local store and purchased their entire flaring kit. While I was there I ran across this handy on vehicle flaring tool and hands down it is the BEST brake line flaring tool I have EVER used. I've done about a dozen flares with this tool and it has done the flare correctly every time so far, knock on (East) wood. I have yet to use the full set but I'm confident it will do every job correctly too

Here's a video of the handy on vehicle flaring tool.....which should have some tips on how to flare with even a cheaper tool.



And here is the large full flaring tool kit from Eastwood.......may be from the same manufacture as John's recommended kit?

https://www.eastwood.com/eastwood-hydraulic-flaring-tool.html

Here's another tool to help make perfect leak-proof flares, it laps the seat after flaring.

What you are trying to duplicate is a partial flare , more like a bubble , to keep the hose from slipping off. Since this the the suction side, I assume, and it's between 2 solidly mounted parts the amount of flare doesn't need to be much.

Is the outside of the tube coated with something? I now always sand the outside of the tubing where you'll be flaring.

No, it's just bare steel. No lubricants or anything on it.

This wouldn't happen to be one of the stainless steel sending units would it ? If so, cutting the tube it with a normal tube cutter will work harden the end. in addition the tubing type should be the annealed type to soften it.

It’s not the sending unit I’m trying to flare. It’s the fuel line where it ends near the gas tank and is connected to the sending unit by a short piece of hose. The fuel line is plain steel, not stainless.

I don't remember seeing a stock fuel line double flared like it would be a for a fitting, I think they were bulged but not double flared
I saw a tool years ago to duplicate that, to bad I didn't buy one then

I bought one that’s sold by several companies. They call it a professional flaring tool. It has a long handle with rotating die set. It works good. The secret to this and most others is they have a longer surface to grip the tubing. Those kind you are using with the clamshell design and the wing nuts works on soft copper and that’s it. I’ve messed up many steel flares with those.

I have acKD flaring tool from NAPA. I've literally done thousands of flares with it, works every time. I had the Mastercool setup but found it too bulky and awkward for on car flaring, the KD is compact and easy to use on car.

There's absolutely no reason or advantage to using a double flare in that application. A single flare is fine. All you need is the OD of the metal line expanded slightly at the end of the line to help prevent the hose from slipping off after its clamped.

It's called a tube beader

https://www.holley.com/brands/earls/products/plumbing_an_fittings_and_hose/plumbing_tools/beaders/

https://www.ebay.com/itm/TUBING-BEA...el-Copper-Nickel-thin-wall-/154055803679

https://www.amazon.com/Tube-Beading...ading+tool&qid=1635684545&sr=8-4

I like all of those. Might get the flare tool from East Wood plus their tube straightener.

Is the tubing your trying to flair metric?

good question
SUGGESTION: You might some scrap pieces of scrap and see if the problem persists.
The only time I've had trouble with steel tubing is when the flaring tool teeth were full of debris. Cleaning with a small stiff wire brush usually fix's the issue.
If the tube has been "scraped enough" you may have to cut a short piece off for a good grip.
only other thing I can think of is you might have gotten an extremely hard piece of tubing (unlikely) or the tool itself is tweaked.
keep us posted

It is a fuel line that I bought from Fine Lines so I would think it wouldn't be metric. Never again will I purchase fuel or brake lines from them. The lines they sent me come nowhere close to fitting. The original fuel line they sent me didn't fit so they had me return it to them along with my original (which of course, fit properly) to duplicate. They then sent a replacement line supposedly copied from my original (and they kept my original) and it was exactly the same as the first line they sent me. So, rather than dealing with them further, I just decided to modify the line they sent me. I've had to cut it twice (now have a coupling in one spot where I had to cut it) and do a fair amount of rebending in the line where it goes up and over the rearend and to the sending unit. Once I get this fuel line issue resolved, I have to deal with the brake lines they sent which don't fit right either.