Best Practices – Sealants

This is the beginning of a new series on best practices in Experimental aircraft construction. In other words, it is a series about the right way to build an aircraft by employing the best products and techniques currently available to us. The goal is to enhance the safety and reliability of our aircraft by building them to the best standards we can. Since mechanical failures in Experimental aircraft far exceed those of certified aircraft, we must admit that we have some room to improve. I hope that this series will be a step in that direction.

Over the course of this series, there may be disagreement as to what constitutes the best practice in some area, or a good product may get overlooked. If you think you have a better idea or a better product, I encourage you to let me know about your concerns. The goal is to pass along good information to our readers, wherever it may come from. In many cases there are also multiple options that are equally acceptable. I will try to point those out as we go forward. Lastly, there are some things that you just shouldn’t do and some products, even though they may be good for other things, that are not acceptable for the task at hand. Red RTV is a good example. It is great for sealing leaks in baffles, but lousy for sealing pipe threads and firewall penetrations.

Not to be lost in all of this is the fact that we are talking about Experimental aviation. These recommendations are not meant to stifle creativity; they are instead meant to help you avoid trouble as you build the airplane of your dreams. Build your creativity on a foundation of good experience and good engineering. That is what “best practices” is all about. Create the aircraft of your dreams, but don’t let the use of ill-suited products or techniques turn your dream into a nightmare.

We begin with sealants. This is a broad topic that ranges from fuel tank sealants to thread sealants to firewall sealants and more, so this article will also be rather wide-ranging. Here we have some great examples of wonderful products that do some jobs well, but are not appropriate for others. Sealants must take into account the parts to be sealed, the medium being controlled, and the environment in which this takes place. Each of these factors bears on the best choice of sealant for a particular job.

Sealants for Fuel and Oil Lines

There are a few important things to remember when it comes to oil and fuel line sealants in airplanes. Use an appropriate sealant on any fittings with pipe threads. These are tapered threads that go into things like engine cases and oil coolers. These threads will have designations like 1/8-NPT or -NPT. The NPT stands for National Pipe Thread. They will leak if a thread sealer is not used. Only apply sealant to clean fittings that are free of grease, oil, or other contaminants.

Do not use sealant on the flare end of AN fittings. These fittings do not seal at the threads, but rather at the flare. Similarly, do not use sealant on Swagelok or compression fittings. The threads do not do the sealing with these types of fittings either. Some fuel systems use these fittings as do most pitot/static systems.

The fittings that allow oil lines to connect to the oil cooler and the engine accessory case have pipe threads (NPT) where they go into the cooler or engine case. Use a thread sealant on these fittings. The sealant should be specifically for use with fuel and oil and it should be designed to work in the temperature range of up to at least 300F. Sealant should be applied to clean threads (male end only) in such a way that it covers the entire diameter of the fitting, but does not come in contact with the end of the fitting or the first two threads. There should be enough sealant on the fitting so that some of it is pushed out as the fitting is threaded in, but not so much that it flows all over the surrounding area. As a rule, most people use too much, but too much can be cleaned up, whereas too little won’t allow the fitting to seal properly. The point that must be emphasized is that no sealant should get inside the fitting or on it in a way that it could get inside. Sloppily-applied sealant can get into fuel injectors, carburetors, or small oil passages and do major damage including stopping the engine.

Of the products currently available, EZ Turn, which is the functional equivalent of Fuel Lube, is a good choice. It withstands temperatures up to 600F, can handle motor oil and gasoline including avgas, and it does not deteriorate in the presence of ethanol. EZ Turn has a proven track record in aviation use, is readily available from aviation vendors such as Aircraft Spruce, and a small container will last you a lifetime. Its only negative is that it is very sticky and can be messy to work with. On the other hand, its stickiness can lend itself to some “off label” uses such as temporarily sticking washers together, sticking a washer to a nut, or sticking a nut to the end of your finger as you reach into a tight spot.

Two other products worth considering are Permatex 59235 and Loctite 567. Both of these products are readily available online, but may be hard to find at your local retailer. Loctite 567 is a great sealant for use with fuel or oil, and it is ethanol resistant. It works at temperatures up to 400F. A small tube makes it easy to apply and will seal more fittings than anyone will have in one airplane. A cure time of 24 hours makes it less than ideal for maintenance use where a quick return to service may be required. Both of these sealants are preferable to EZ Turn for high-pressure applications such as brake line fittings.

Permatex 59235 also works well for aircraft assembly. It is fuel, oil, and ethanol resistant, and works at temperatures up to 400F. Parts can be repositioned for up to 24 hours after initial installation due to its longer (72 hour) cure time. This makes it great for initial assembly, but not as good for maintenance where such an extended return-to-service time may be unacceptable. It too comes in a small tube that makes it easy to apply and will seal more fittings than most airplane builders will ever encounter.

Please note that pipe dopes and thread sealants made for water pipes such as those available at your favorite home store are not appropriate for aviation use. They are designed for water pipes, not fuel and oil lines. I must also mention that RTV sealant is not a thread sealant. It should not be used in fuel or oil systems for sealing threads or anything else. Safety dictates that you use a product that is designed for the task at hand, not just whatever is handy at the moment. Please use the right product for your application. When in doubt, call the product’s technical support help line.

Before we leave the topic of thread sealants, we need to talk about Teflon tape. My initial thought was to simply say, “Don’t use it on, in, or near an airplane, ever.” The reason for that is that it is so easy to misuse, leaving a tiny strip of tape in a place where it can clog a fuel line or oil line or lodge in a fuel-flow sensor or carburetor. But there are some aviation vendors, Pacific Oil Cooler for one, who routinely use Teflon tape, so it obviously can be used appropriately. The important thing is to never let the tape cover the end or the first two threads of a fitting. The bottom line is, you can use Teflon tape, but there are better choices available to you, so save the tape for your plumbing around the house and use something else on your airplane.

Sealants for Engines

In every case, the engine manufacturer’s overhaul manual or other relevant service literature should guide your decision regarding the proper sealant to be used for assembling and/or replacing various engine components. For example, Lycoming has very specific recommendations for sealing engine crankcase halves and the crankshaft oil seal (see sidebar). These recommendations should be heeded. Substitute products may or may not yield suitable results. Don’t even think about assembling an engine without the overhaul manual.

When applying sealant to engine parts, especially any parts that will be inside the engine case or exposed to the inside of the case indirectly, it is vital to prevent excess sealant from intruding into the internal spaces of the engine. Excess sealant can clog oil passages and lead to engine failure. This has happened more often than you might think.

Sealants for Firewalls

The firewall between the passenger compartment and the engine compartment has a vital function, namely to keep the heat and flames of an engine fire away from the occupants of the airplane for as long as possible. The FAA says a firewall should contain a 2000F fire for at least 15 minutes. That is why most firewalls are made of stainless steel instead of much lighter aluminum. To maintain the integrity of such a firewall, a sealant rated for 2000 is also required. This means that sealants such as Dow Corning #736 Red RTV are not going to get the job done because they are rated at 600F maximum and 500F continuous. Red RTV has its uses, but this is not one of them.

Recommended sealants include FlameSafe FS 1900 and 3M Fire Barrier 2000+. These sealants are rated for prolonged exposure to fires. Aircraft Spruce carries the 3M product and Glasair Aviation specifically recommends and sells FlameSafe. Be sure to clean surfaces thoroughly with MEK before applying these sealants. Needless to say, use proper precautions for handling solvents and sealants such as these, including proper eye protection, gloves, and good ventilation. Always consult the manufacturers’ literature for proper use and handling, including proper surface preparation.

Flamemaster CS 1900 was once popular with many builders, but it has become hard to get, and honestly, it is not rated for continuous exposure to 2000. On that basis I would not recommend it. However, I am not here to pick a fight with any kit manufacturer. Carefully consider the recommendation of the maker of your kit and then make an informed decision after consulting the relevant information.

For those of you building aircraft that you wish to register under the ELSA rules, you must use the products approved by the manufacturer and should get their permission before using any substitutes.

Baffle Sealants

At last we find the proper use for Dow Corning #736 Red RTV. Sealing baffles in the engine compartment is a job made for red RTV silicone sealant. It is easy to apply and hardens fairly quickly. It sticks well to baffle material, engine baffle seals, and engine cases if the surfaces are clean. Use it to seal small gaps and plug small holes in baffles and seals. A wetted finger will work nicely to smooth out freshly-applied sealant. It can be messy though, so mask off areas to remain clean and be careful. Use plenty of paper towels to wipe up excess material as you go. As with other sealants, be sure all surfaces are clean before applying. Red RTV has not held up well over time as an adhesive to bond baffle seal material together, so it is not recommended for that use.

Fuel Tank Sealants

PPG Pro-Seal PS 890 B2 is the standard aircraft fuel tank sealant in use today. Van’s Aircraft recommends and sells a similar product (Van’s part number MC-236-B2) by Flamemaster. Both products are two-part sealants specifically designed for the construction of wet-wing fuel tanks in aircraft. They do an excellent job if properly applied, but careful surface preparation is of paramount importance. Good ventilation, eye protection, and gloves will make the application safer and cleaner. By the way, this stuff gets everywhere, so old clothes should be worn. Surfaces must be absolutely clean and unoxidized. Remove the protective coating from Alclad aluminum no more than three days prior to sealing if possible. Clean the surfaces to be sealed with acetone or MEK immediately prior to sealing, and use a non-lint-shedding cloth, not paper towels. Use enough tank sealant so that it squeezes out along the entire length and both sides of each rib, but try not to have large amounts of extra material that will only add to your cleanup. Refer to Van’s instructions for applying fuel tank sealant if you are building an RV-type aircraft. They have a lot of experience with this material and should be regarded as the best source of information about its use.

By the way, fuel tank sealant is not suitable for sealing firewalls—it’s flammable. Nor is it suitable for any other use in the engine compartment.

Sealant and Adhesive for Windows

Glasair Aviation recommends GE SilPruf SCS2000 sealant and adhesive for installing windows and windscreens in their aircraft. It is also useful as a general-purpose sealant for places such as the intersection of the fuselage and gear legs. It remains flexible over time and comes in a number of colors, but it is generally not paintable. Consult Glasair for more specific instruction on using SilPruf for installing windows. It has a long and excellent track record in Glasair airplanes. As with any of these products, the use of expired SilPruf is strongly discouraged.

General Remarks

There are many fine sealant products on the market today. Some of them are particularly well-suited for use by aircraft builders. However, a product that may be fine for one job may be exactly wrong for another. Take the time to do the research required to identify the right product for the job you are doing.

Sealants have shelf lives that are limited. Never use a sealant that has gone past its labelled shelf life or has been improperly stored. Failure is likely, and it will be your fault. Discard any expired sealant products immediately to avoid inadvertent use. Surface preparation is vital for the successful application of any sealant. Consult the manufacturer’s literature to see what cleaning solvents work best with the product you are using. Generally, a volatile organic solvent such as acetone or MEK will work well, but do not assume that these will be best in every case. Read the literature and see what is recommended. All of this material is available online to anyone who wants to look for it. If you are struggling to find the right product or its proper use, don’t be afraid to email or phone various sealant manufacturers. It has been my experience that they are usually more than happy to help steer you in the right direction.

Eye protection, disposable gloves, and good ventilation are always a good idea when using any chemical such as these sealants. For safety’s sake print out the MSDS for each product you bring into your shop or hangar and put it into a notebook you can keep handy. Quickly read it over before you file it away to see what the exposure risks are and how to deal with them if they occur. You will probably never need to use it again after that, but if something happens this information can be very valuable to a first responder who may come to help you in case of an accident.

Sealing a Lycoming

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Lycoming has specific recommendations for sealing engine crankcase halves and the crankshaft oil seal. These can be found in Lycoming Service Instruction 1125D and 1324C. Key points are covered below. However, you should refer to the actual Lycoming instructions for complete information.

Lycoming Crankcase Sealants—From Service Instruction 1125D

POB No. 4 is the preferred sealant for Lycoming engine crankcase reassembly. It should only be applied to one half of the crankcase. Apply a thin film of POB sealant on the outside mating surfaces, as shown. No build-up is permissible. Visibly, where sealant is properly applied, it will produce a wet appearance on the machined surface. If it does not have a wet appearance, reapply. Note that this sealant is used primarily to hold the silk thread in place. Be certain to wipe clean all inner edges of any excess sealant. Also, ascertain that no sealant has come in contact with the bolt holes or any surface other than the shaded areas shown.

RTV-102 or Loctite 515 may be used as the only approved alternates to POB No. 4. They too are to be applied to only one half of the crankcase. The application instructions are the same, except that no silk thread should be used.

Lycoming Crankshaft Seal Sealants—From Service Instruction 1324C

When installing or replacing the crankshaft seal on a Lycoming engine, the surface of the engine case where the seal will sit should be thoroughly cleaned with acetone or MEK solvent. After it is cleaned, apply a layer of Dow Corning 737 Neutral Cure Sealant prior to installing the seal. Pliobond #20 may be used as an approved alternate. Dow Corning 737 requires 24 hours to cure, but Pliobond cures in 15 minutes.