Sealants for fuel fittings, fuel cap neck & sender

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.


Larry Zepp

Zodiac CH 650B builder

Fort Wayne, IN 

Views: 1697


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Comment by Larry Zepp on April 27, 2013 at 9:58am

Hi Dave,

Yes, heat is a good option. Our standard method to release 600 series high strength Loctite at work is torch or heat gun to get the fastener to 400 F for at least 15 min. After that time Loctite has lost much of it's strength. For removing a threaded fuel cap, I think I would use the heat gun held back a ways instead of a propane torch !!!

Best Regards,  Larry Zepp

Comment by David Gallagher on April 26, 2013 at 11:00am



Your temperature comment reminds me of when I told another local EAA'er about having to destroy my fuel cap filler necks to get them out.  He asked, "Did you try heating them to soften the sealant?" Crap!, Damn! Why didn't I think of that !?!? I just may have worked.  Sometimes we get to tunnel vision'd to think of alternative ideas. At least I did.



Comment by Larry Zepp on April 25, 2013 at 8:22pm

Hi Dave,

Thanks for your comments on don't use too much! And Permatex #2 is supposed to be "non-hardening". I don't want to have leakage into the wing, so I will limit it to a few threads. One possible advantage of using the polysulfide rubber is that it gives up some strength at about 300F. Best Regards,  Larry Zepp

Comment by David Gallagher on April 23, 2013 at 11:07am



I only have one comment for the fuel cap threads....don't over do whatever sealant you put on.  I used Permatex #2 and when I had to pull the wings apart for the "B" mods, the caps would not budge.  There is a lot of thread surface area, so even a little sealant goes a long way to cementing them in place.  I ended up having to cut the caps off.  I am sure just a thread or two worth of sealant at the end of the threads is enough since they are not holding fuel in, just occasionally being splashed by fuel when the tanks are full.  I hope to never have to pull the tanks again, but I would rather pull them than then have to replace a leaky fuel sender through the small access hole.


My $.02.  Good luck,



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