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When I did my tubes, I used a torch and tried to limit the heating to the last 1/4" or so of the tube. If I remember correctly, it doesn't matter if you let it air cool or quench it with water after heating. I used the modified air hammer and found that if I heated too much of the end, it would produce a "bulge" in the tube about an inch in. Don't know if this is an issue with using the tool manually or not.