Fuel Tanks – Modern Fuels – Thread Lockers and Sealants - In Need of Some Advice


I am building a CH750 in the UK –  and am ready to install the fuel tanks into the wings.   However, I am now full of doubt as to what ‘thread locker’ to use on the finger screen and nipple fitting and what ‘sealer’ to use on the screws that locate the fuel senders into the tanks.   I am thinking that any leaks in the wing tanks can only mean one thing - and that’s opening up the wings!   It seems that with modern fuels the way they are – with increasing amounts of ethanol/alcohol in them – most of the test results on sealants and thread lockers are suspect.   For the last few weeks I have been experimenting with two of the most well-known brands, Locktite and Hylomar, with various specimens in glass containers full of either MOGAS or AVGAS.   Interestingly, the thread lockers Loctite 577 and Hylomar 4430 seemed to be okay in both MOGAS and AVGAS, but the gasket type sealants, ie Loctite 5922 and Hylomar Universal Blue, totally disintegrated after a week’s immersion.  Interestingly also, when I had previously phoned up Loctite and Hylomar - I was assured that their products had been fully tested with modern fuels and all  their products were ideal for use in fuel systems.   As I see it - not quite true!   So, should I use the thread locker only and just rely in the rubber seal for sealing in the fuel tank sender units?   Is there a more suitable sealant out there?   Perhaps I ought to use Teflon/PTFE tape for the finger screen and nipple connection ...... at least I would get a good seal, if not locked threads?  I would be most grateful is any anyone who has successfully passed this building stage could tell me what they used/how they got over this problem.

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Tony - As far as the sending units the rubber gasket worked fine for me; 4 years and counting. Teflon tape on the finger screen is fine the taper (tapered pipe thread) is what does the locking. I had originally used a NAPA liquid type thread sealer (dark brown in color can't remember the name) recommended to me by a good friend who's a car guy. Did not work!.... I recommend rubber gasket alone (make sure tank surface is clean) and Teflon for threads.


Hi Chris - Many thanks.   This is really useful.   I was coming round to the use of Teflon (PTFE) tape - as had found it identified on a website that drew on material by the Federation of British Historic Vehicle Clubs, who identified Teflon as a material "recomended".." for use in Ethanol and Ethanol /Gasoline Blend Aplications".   So, this makes great sense.     Also, the fact that - if the rubber seal is correctly located and 'clamped, there should be nowhere for the fuel to leak through....although this assumes that the rubber seals themselves do not degrade over time.   I should have thought that Zenith themselves have thought through this.   I am really grateful for this advice.   I was getting a little overwhelmed by the 'bad' advice I was receiving from so many so-called 'experts'.   Also, it seems to be an area where everyone is a bit at sea.    I realised this when my very excellent Inspector said that I was to make up my own mind what to use - as all the test data on seals and thread locking materials was virtually null and void with the changes in fuel constitution.     All the best, Tony Oliver, Essex, United Kingdom.

My two cents -- Bare rubber gaskets for the fuel qty sender, Teflon paste for the threaded connections. The teflon tape, if installed incorrectly, can result in shredded and torn bits of the tape loose inside your fuel or oil system. The teflon is mainly to lube the threaded connection as you screw it toghether, not to seal it. If it is a pipe thread (tapered) the act of screwing the slightly tapered fitting into the hole until it is a wedge fit pretty much seals it up (or at least, it is supposed to). If it is a straight thread, there should be an O-ring or gasket or some such seal built into the joint already.


This is the stuff I usually use - lubes the threads during assembly and helps to seal the joint. Use very sparingly, it does not take much and you do not want the surplus wandering around inside the engine. I use it for automotive and aircraft connections, oil, fuel, air, vacuum, whatever. http://www.aircraftspruce.com/catalog/cspages/permthrdseal.php?clic...


The usual caveat, I am not a certified mechanic or a "guru" but this has worked well for me. I got exposed to it when it was provided in an STC kit to install some oil lines in an engine/oil filter installation. I liked it and have used it for many years since.

Hi Bob -  Great.  This is really useful.   I was coming round to the use of Teflon, at least,   Teflon has been identified on a website that drew on material by the Federation of British Historic Vehicle Clubs, who identified Teflon as a material "recomended".." for use in Ethanol and Ethanol /Gasoline Blend Aplications".   So, it's great to hear from someone who has used it in the field.     Also, you have confirmed that bare rubber is the answer.   As I told someone else who replied to this theme, I was really getting a bit depressed/overwhelmed by the 'bad' advice I was receiving from so many so-called 'experts'.   Both Loctite and Hylomar said when I phoned them that their materials were fine with the 'new' fuels.    When I put specimens of the gasket sealants (Hylomar Universal Bluue and Loctite 5922) into MOGAS and AVGAS the specimens totally disintegated.    It seems to be an area where everyone is a bit at sea as - it seems that all the test data on seals and thread locking materials - now that alcohol is in petrol in increasing quantites - is virtually null and void.     Best regards, Tony Oliver, Essex, United Kingdom.

I agree with Bob - Teflon tape is generally frowned upon in aviation fuel systems due to the possibility of getting tape debris in the fuel line. Some argue "I'll be careful and not get it near the end of the fitting." However, if you ever remove that fitting, sometimes fine bits of the tape remain in the threads and get pushed into the fuel line the next time a fitting is screwed in!

I put a very light coat of the sealant that I ?think? Zenith recommended - "EZ Turn" - on the sender gaskets and the threads of the sender screws. Not a drop of leakage so far at 66 hrs! By the way, I also used stainless braided AN lines from tanks to firewall and no leaks and not even a hint of fuel odor ... and those fittings don't use any sealant!

EZ Turn is actually a fuel proof grease. Its main purpose in life is to lubricate older style metal to metal fuel selector valves. Hence, the name implies that if you use their fuel proof grease your fuel selector valve it will turn easily. Since it is a goopy grease and since it does not dissolve in fuel or oil, it will provide some sealing function. Also, since it does not dissolve in fuel or oil, you need to be sure not to slop so much of it into the joint that some oozes out into the tank or plumbing -- it will remain as undissolved globs and can migrate around and clog up small openings such as carb jets and the like.


By the way, if you are trying to lube up a joint for assembly (like slipping a hose over a fitting) and do not want the chance of globs causing problems, use Vaseline. That dissolves very nicely in fuel and oil so you get no globs circulating. You also get no sealing action, unlike a fuel resistant grease. Also, the joint will retain a bit of the vaseline and be kind of slippery, which increases the chance of a hose later slipping off a fitting. I only use the vaseline trick as a last resort, basically almost never.

Hi Bob (again) - Thanks for this.   It's certainly helping me find my way around what for me has proved to be difficult territory.  Again - much appreciated.    Tony Oliver

Hi John - Thanks for your points – which I read with great interest.   I take your point about Teflon/PTFE and the need to be careful.    I have always been mindful of the need to avoid wrapping it on the bottom thread(s).     You have educated me on EZ Turn, which I see Bob Pustell has further commented on.  I was very interested in your comment about the use of “stainless braided AN lines”.  The fuel line supplied with my kit was rather heavy-looking black rubber hose (identified as 3/8 in (3.52mm) 10.SAE-30R7-KX078698508818 – Made in USA 03 24 10).    Certainly, if you can spare the time I should like to heard about your AN lines.   Up to this point I was making very good progress with my CH750 kit (the first in the UK) and having built the rudder, horizontal stabilizer, elevator, slats, flaperons, and carried out lots of etch priming and instrument panel work, I have been working on the wings.   I have got to the point of getting fuel tanks ready to insert into the wings – and only the top skins to go on.   It has rather hit me between the eyes that with modern fuels the way they are – with increasing amounts of ethanol/alcohol in them – most of the test results on sealants and thread locker are suspect.   For the last few weeks I have been experimenting with two of the most well-known brands – with surprisingly bad results.   So, with the price of getting it wrong very high – ie leaks could mean dismantling wings, I have come to a grinding halt until the whole issue of tanks, sealants, thread-locking, etc, has been sorted.   Your input very much appreciated.   Tony Oliver, Essex, UK.

My A&P friend who has built several planes recommended the braided stainless AN fuel lines. They are Teflon-lined and impregnated with carbon to make the line conductive to avoid static build-up. Don't use the cheapy Chinese import lines ... they usually don't have the carbon - you can tell by looking at the Teflon liner: White = no carbon, Black = carbon.

The most expensive source is aircraft suppliers such as Spruce. Many, including my A&P and myself, have used Earl's from ANPlumbing.com . They are the "industry standard" for the racing/dragster crowd. If these lines can stand up to the witches' brew that the nitro-methane dragsters use, it'll handle a little ethanol with no difficulty! Earl's brand is still expensive, but great quality - you'll easily spend $200-300. The "Speed-Flex" AN6 is what you want for fuel lines and use the aluminum "Speed-Seal" fittings. I had to use one steel fitting where the line joined a stainless gascolator mount. (In other words, steel-to-steel and aluminum-to-aluminum.)


The ANPlumbing site has videos on how to install the fittings. However, if you want to use smaller holes to pass through tight areas (you still should protect the line with a grommet), you may want to pass the line first and install the fitting once the line has been run - it's a little harder to do, but do-able! Like all things, just takes a little advance planning. The lines accept about a 4" bend radius - it's just flexible enough that I could run my lines down the uprights (I encased them in plastic loom just to provide a little extra abrasion protection - however, I'm sure the braided stainless would fret the aluminum and not the other way around!) and through the central tunnel to almost totally enclose and hide them. Use padded Adel clamps to secure the lines. There are numerous adapters available to convert to other plumbing in the line such as the fuel screen, gascolator, etc.


Like I said - no sealants, no leaks, and not a drop of fuel or whiff of fuel odor in the cabin! Some examples:

Hi John  - Many thanks for such a great explanation.   Thanks also for the excellent pictures.     While this is an offshoot from where I started, ie  airing my difficulties in selecting appropriate thread lockers and sealants which will stand up to fuel with increased ethanol /alcohol  content – this has started me thinking about my fuel system-to-be.   I certainly like the sound of the braided fuel hoses, and when I thought about it, it occurred to me that I do have a couple of short sections of such hose in my Long-EZ.    The fuel hose and jubilee clip fittings at the fuel tanks that have been supplied with the Zenith kit do look a bit cheap, and I think you are really right in your assessment that lines, etc, used by the “racing/dragster crowd” could provide a logical way to go.   At least, such braided Teflon-lined lines – at least - where the lines need to be encased in the wings, could prove a real advantage.    I really don’t want to have to have to dismantle my wings following any leaks in the future - so this could be extra insurance.   So – next week I will be paying a visit to some hose and fittings specialists (that supply the sort of fittings you describe) to see what they recommend.  Many thanks again for the write up – it has been very, very useful.  Tony Oliver, Essex, United Kingdom

Your welcome! By the way - I neglected to mention that I ran one continuous line from each tank to a bulkhead fitting at the wing root (so it could be disconnected if the wing ever had to be removed), then one continuous line from each root to the Andair valve (so there's no "hidden" connections in the uprights or tunnel that would be difficult to access later), then one line from the Andair valve to the gascolator fitting on the firewall. My intent was to minimize as many connections as possible within the fuselage/cabin.

I realize the single gascolator is "not quite" at the lowest point in the system, but I look at it primarily as a filter. My avgas is stored indoors and re-filtered through a filter/gascolator prior to pumping in the plane, the plane is 100% hangared, so I'm not too concerned about water - never had a drop in 20+yrs of using this system. I like having the two, independent lines to the fuel valve from the two tanks. If my fuel was of questionable sources or I was tied-down outside, etc., I'd probably put a gascolator at the low point on each line, but that's a lot of plumbing! LOL!




Hi John – Yes, many thanks for this.   I printed out your earlier pictures and can I see how you have done it – and I must say, it looks a pretty impressive bit of work.   In thinking about your earlier comments I had decided to see if I could go the braided fuel line route.   Accordingly, I put out another request for advice, this time seeing if I could work out where any ‘slack’ fuel line needs to go - as I am building a CH 750 with the wing fold option - so (as I see it) the wings will be close to the top of the fuselage when in ‘flying mode’ and (again, as I see it) about 12 to 15 inches away from the fuselage, when in the ‘wings folded’ mode.   The distances are my guess (only) at this stage.   I had posted this message before your second message arrived.    I am presuming that at least a few builders have incorporated the wing fold mechanism.    Personally, I have been intending to use the wings fold option often - as I might even keep the aircraft at home on occasions and tow it to an airfield when required.  It would seem from looking at your pictures - that the only way in which I could do this is to have quite a bit of slack in the fuel line and this would enable the ‘excess’ line to ‘slide’ into the fuselage (perhaps through a large grommet) when the wings are in their flying horizontal positions?   All the best – Tony Oliver, Essex, United Kingdom


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