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Is it possible that you have 1/4" steel fuel line. The ferrule will not compress steel fuel line. The ferrule and the nut are brass so it should not take a lot of force to compress the ferrule on copper or copper alloy tubing. Excess force will round off the nut. You might look into a line wrench specifically made for this application that will grip the nut better so there is less chance of rounding the new nut off. Frank