The 1/4" side is notched, too smooth to trust. I was able to easily hand thread it into a 1/4 ID fuel line (Gates 1/4" Barricade Fuel Injection Hose) insert and remove without twisting or twisting. While this product may be useful for other applications I would not trust it for fuel lines, low pressure or high pressure applications. The 5/16 inch seems to be the same. I really wanted this to work as I like to use stainless steel whenever I can. I've read others with similar complaints but had to check for myself. Please do not use these for fuel lines.
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