Every inverted flare nut I have worked with for brake or fuel is a soft ferrous/steel, those are factory type nuts. If you use aftermarket items, aluminum, brass, stainless steel, are all available but they are not designed for Bundy or copper/nickel.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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