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Slip the flaring tool over the clamp and place the anvil disc over the end of the tubing. Hold everything in place while you tighten the bolt in the flaring tool. At this point, you will tighten the bolt until the anvil disc is pressed completely flush to the clamp. You won't be able to see any portion of the tubing at that point.
Remove the flaring tool and the anvil disc. The flared end of the tubing should be well formed and even all the way around. The picture above is not a perfect example of what it should look like. This one was slightly off center because the end of the tubing was not cut squarely. As you can see, it caused the initial flare to be slightly formed off to the left side. Anyway, the above picture should give you a pretty good idea of what you are looking for. Nicely shaped, even all the way around, and properly centered on the end of the tubing.
Put the flaring tool back onto the clamp and once again tighten
it. You are now applying the 2nd flare to the tubing. Tighten the
bolt until it does not go any further.
Here is a close-up of the 2nd flare being applied. The bolt has been tightened as far as it can go.
Here is the final product. This is the same one that was slightly off center after the first flaring step was completed. This has a slight ridge on it and won't provide a good seal. Because the end of the tubing was not squarely cut, it resulted in this imperfect flare.
Well, you've seen an example of not what to do....so obviously, you should have no problem identifying a good flare. It really is not that hard to do. Scott has gotten more of them correct than he has messed up so he is batting better than 500 (a good score in anyone's book). You can expect to spend around $35 and up for a good double flare tool. For occasional use, you might find it difficult to shell out $75 or more for a quality tool. But I would not recommend getting one from Harbor Freight either.
Good luck on your projects and remember to practice on a section
of tubing before you do it for keeps. You can get a tubing at your local
auto parts store. A 2' long section is plenty cheap and you will get a lot
of practice from it.