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I am finally working on the rt. tank and fuel system prior to closing the right wing. I did some research on sealants for the stock welded fuel tanks - so I should never have any leakage problems!!
Tapered pipe thread fittings -
I have long experience using either Loctite # 592 - Pipe sealant with Teflon (PST) or Permatex pipe thread sealant on fuels, oils and high pressure gases. Both sealants cure when air is excluded and have teflon in them to prevent galling of metal pipe thread surfaces as they are wrenched together. I am using the Permatex just because it is more available.
Fuel cap straight thread sealing -
Selecting this sealant is more of a challenge - due to this being a large straight thread and the possibility of an ethanol percentage in any mogas fuels. The best rated material is a two part polysulfide rubber sealant. This is extremely resistant to gasoline, ethanol and many other chemicals. Standard trade names are Pro-Seal, 3M aerospace sealant, and other brands. This seems like a much better choice than single component sealers like Sealube or Loctite sealants.
This material has a strong bond to metals and remains flexible after cure. For the large dia. threaded cap neck, you should avoid using a single component polysulfide sealant because it needs moisture to cure and the thread will have very slow access to the surrounding air.
I am using the 3M material AC-340-B2 two part polysulfide rubber sealant from Aircraft Spruce PN 09-02638. This mtl. will not sag on vertical surfaces and will trowel onto the thread in a thin layer before assembly after the tank is riveted into the wing. I will also use a film of this same sealant on the fuel sender screws and gasket.
Zodiac CH 650B builder
Fort Wayne, IN