Best fuel line thread sealant?

[Steeveedee]

I'm not aware of a conical washer that is made for inverted flare fittings. The only ones I've ever used were for spacecraft fuel systems, where single flares were used. That use comes from aircraft use. I think that they (single Vs double flares) have a different flare angle, but I'm willing to be educated about that. The thing with copper is that it is malleable enough that it would conform to the slightly different flare angle. I would like to point out that if you use a copper conical washer, it is one-use only. Don't try and save money, for what they cost. Of course, at 7 PSI they'll probably seal again, But a spacecraft fuel system typically runs about 600 PSI. Aircraft hydraulic systems run much higher, and those washers just get tossed after removal.