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A Few Engine Compartment Notes

By Tony Bingelis (originally published in EAA Sport Aviation, November 1996)

If you are a typical homebuilder you probably take a good look at your engine compartment and engine just about anytime the cowling is off . . . a good habit to develop.

Why miss an opportunity to check that everything is O.K. ... that nothing has failed and that none of the components are loose or show signs of possible failure and, of course, that the various hoses and lines are not rubbing anywhere. It only takes a few extra minutes.

At any rate, I think we ordinarily do a good job of checking the general condition of our aircraft frequently. However, there is more to consider, especially after the aircraft is flying and its flight hours are beginning to add up.

Think back. Are you still satisfied that the various engine compartment components and parts you originally selected, and the installations you initially made during construction, will safely withstand the heat and vibration sure to be encountered during the aircraft’s service life? Everything in the engine compartment must endure severe conditions associated with high temperatures and vibrations. Nevertheless, we sometimes forget this during construction and perhaps substitute materials and hardware that might not be exactly the best for the job.

The FAA mentions Improper Installation in its Appendix D to Part 43 which details items to be included in annual inspections of aircraft. What constitutes an improper installation? Obviously the FAA does not, cannot, go into detail in that regulation and assumes that the individual making the installations and making the inspections has the necessary experience and technical knowledge to do it properly.

Having said that, how would your various installations check out in the following specific areas:

    1. Battery for improper installation

    2. Exhaust stacks for improper attachment

    3. Studs and nuts for improper torquing

    4. Engine controls for improper travel, improper operation, and improper safetying

    5. Lines and hoses for improper condition

In each instance do you know what constitutes the proper installation?

Now might be a good time for you to restudy these specific installations in your own engine compartment.

Although what you have may be functioning as expected, some of the installations might not survive for long should unforeseen adverse operational conditions arise. Inflight difficulties are often compounded when aircraft quality materials and the correct hardware is not used, and proper installation procedures are not followed.

In this regard, I offer a few comments based on my notes culled from those recently made while looking over a variety of projects under construction and a number of operational homebuilts caught with their cowlings off.


I am convinced that all hoses and lines forward of the firewall carrying flammables (fuel, oil, hydraulic fluid) should be fitted with firesleeves prior to installation. These sleeves will make your fuel and oil line installations virtually "fireproof."

Incidentally, the FAA defines a fireproof installation as one that can withstand a direct flame for 15 minutes under operational conditions without failure.

A firesleeve is just that, a protective sleeve that can be slipped over a fuel line or oil hose to insulate it against exposure to heat and flames. A firesleeve has a smooth red outer skin over a cushion-like asbestos inner liner.

Select the firesleeve size to match the hose diameter to be protected. For example, although a flexible -6 oil hose only has a 3/8" I.D., its outside diameter is much larger. Nevertheless, the proper firesleeve size will be a "dash 6" whose inside diameter will naturally be large enough to slip easily over any type of -6 flexible hose.

Cut firesleeves with a large pair of scissors or with a razor blade. Cutting them on a bandsaw will pull and distort the fire resistant (asbestos) inner liner. Cut a firesleeve so it is enough to cover the entire length of hose but not the metal hose fittings.

After the firesleeve is slipped over the hose, each end of the sleeve should be secured to the encased hose assembly with a firesleeve clamp. This is a 1/4" stainless steel band some builders find difficult to tighten. They, therefore, substitute the heavier and wider standard stainless steel hose clamps that are available in most hardware stores. The most common variety is easily tightened with a screwdriver.

At any rate, do not use plastic tie-wraps to secure firesleeves to a hose assembly.

A less expensive but acceptable method of securing the firesleeve ends is with a couple of wraps of .030" stainless steel safety wire. It may not look as nice as a stock steel snap strap clamp but it is an effective method.

EAA member Steve Walton, Hollywood, FL, shares the following little known information regarding firesleeve installations.

Because aircraft engines sometimes get dirty and oil covered (gosh! . . . not homebuilders’ engines!), he points out that the proper installation of a firesleeve calls for dipping the firesleeve ends in an appropriate sealer to close the ends of the inner fabric to preclude the "asbestos" liner from a tendency to wick and collect oil (that’s mighty flammable stuff) and dirt over years of service. He feels that this soak-collecting of oil of the inner lining, over time, makes an unsealed firesleeve more dangerous than an unprotected hose in the event of an engine compartment fire of any nature.

The two firesleeve fireproofing end dips he suggests are Stratoflex Part No. 5027 and Aeroquip End Dip CD-1. Each costs about $50 per quart, but they should keep well because they do have good shelf lives. He points out, too, that a quart could be shared by a number of builders to cut the cost.

However, knowing my readers, I believe most of you would balk at the cost. Still, sealing the firesleeve ends is a good precaution. Perhaps using that less expensive red high temp (up to 500 degrees F) RTV sealant like Silastic 732 or Dow Corning’s high temp RTV Silicone Rubber Adhesive Sealant (available at most auto parts stores, $7-$9 for a 3 oz. tube) would be a more acceptable alternative.

It certainly would be easier to apply and sealing the ends of the firesleeves could be done as a retrofit on the aircraft without disconnecting the hose assemblies.

Appearance Matters Too

Good appearance is often a fringe benefit of a good quality installation. Here are a few examples of what I mean:

    1. Safety Wiring - When safety wire is used try to make the twists uniform and not overly tight. Cut the wire ends about 6 twists beyond the part being safetied. Be sure the sharp twisted cut ends are curled under with a pair of needlenose pliers to keep them from inflicting harm to exploring hands during some future inspection.

    A pair of safety wire twister pliers is worth acquiring. The tool will help simplify safety wiring in cramped locations and ensure a quality job with snug uniform twists.

    2. Primer Line Installations - Long runs of 1/8" copper tubing, as installed in the typical primer system, generally have a 2" loop formed in the line to better absorb vibration and minimize the likelihood for the tubing to become brittle and break after prolonged exposure to operational vibration.

    Some of those formed tubing loops looked very crude. It is not difficult to form the soft 1/8" diameter copper tubing into a nice uniform 2" loop simply by forming it around a round object like a short section of pipe or a small jar. Also, for a professional looking job strive to install the small diameter tubing with smooth even curves blending in with uniformly straight stretches between supports.

    3. Stainless Steel Clamps - Cut and trim the excess ends protruding from stainless steel clamps used to secure intake hoses, ducting, heat shields, and similar applications. File the sharp cut edges smooth.

    4. Engine Mounts - I still believe it is important that an engine mount be painted some light color so that defects and cracks would be easier to detect should such problems develop. The paint recommended is a good grade of epoxy or, perhaps, an acrylic or polyurethane enamel which will not easily soften and wipe away when lightly cleaned with lacquer thinner.

    The paint job on most engine mounts suffer nicks and scratches long before the engine installation is finally completed. Take a few minutes to touch up these eye sores with a small camel hair paint brush to keep your engine mount looking good.

    5. Tie-Wraps - Many builders depend on tie-wraps as a simple means for securing fuel lines and oil hoses, ducting, primer lines, senders and other engine compartment parts to the engine mount and to any other convenient nearby component or structure.

    The more engine installations I view the more convinced I am that many homebuilders have gone overboard in the use of plastic tie-wraps in the engine compartment. After all, plastic tie-wraps are not fireproof nor are they fire resistant.

Since tie-wraps and WD-40 have appeared on the aircraft maintenance scene, maintenance work has been simplified but not always to the good.

Although it is considered to be O.K. to tidy up loose wires in the engine compartment by tie-wrapping them together, it is not O.K. to rely on tie-wraps to mount and support essential components and parts because a broken exhaust pipe or an engine compartment fire could quickly melt the tie-wraps and compound an already serious situation.

Save your tie-wraps for simple wire bundling. Even then, wire bundles and other components may be better secured with a few fire resistant loo-type cushioned metal clamps.

What About Harmonic Dampeners?

Ever notice how abruptly a wood propeller stops spinning when the engine is shut down? Why is that?

Well, a typical wood propeller for the average 4-cylinder Lycoming aircraft engine weighs less than half that of a metal propeller and, therefore, lacks the "flywheel effect" of the heavier metal prop.

The installation of a harmonic dampener has the effect of increasing the up-front weight of a wood propeller installation to the equivalent of a metal propeller installation.

Several local builders, myself included, have replaced our modified used metal propellers because of their dubious histories and because we were scared of them. The new wood propellers did give us peace of mind but most of us really miss our metal props.

A couple of stubborn gents resisted the switch to wood props and ordered new Sensenich metal propellers even though they were aware that they are not approved for operations at the maximum 2700 rpm for which the Lycomings are rated.

One builder installed a harmonic dampener for his wood prop installation and was so pleased with the results that I also bought one for my RV-6A.

The claims made for a harmonic dampener are that it will dampen out vibrations by 30 to 40%, increase rpm, extend engine life and improve engine starting and idling. I don’t know how valid their claim is for extended engine life but the other claims seem to be fairly accurate for my installation.

Although you do have to remove the propeller to install it, the installation of a harmonic dampener is uncomplicated - simply bolt it to the starter ring gear with the 14 bolts provided. Just insert them in the existing holes in the starter ring and torque the nuts.

If you have a wood propeller and a slightly tail heavy tendency you might benefit from the installation of that fairly heavy (approximately 12 pounds) harmonic dampener.

As you may have suspected, a harmonic dampener is not inexpensive (approximately $350) but it is a beautiful piece of machine work and cost seems to be reasonable in view of the claims made for it (Mark Landoll Starter and Alternators, 405/392-3847).

Baffling Options

Air cooled aircraft engines depend on one of two different baffling systems for engine cooling. The most common of these is the one where metal baffles are attached to the engine forming the side walls and back wall creating a pressurized air chamber above the engine. The top of the cowling serves as the third side of the cooling chamber. The upper edges of these metal baffles are sealed against the cowling with rubber-like strip seals riveted to the baffles. This method of baffling the engine makes it imperative that none of the incoming cooling air can escape past the baffles without first passing down through the engine’s cooling fins.

The second more efficient method relies on a completely enclosed pressurized cooling chamber independent of the cowling. It generally produces more cooling with less cooling drag than the other installation.

In a tightly cowled engine an enclosed compression chamber is more difficult to install unduly limiting access to the spark plugs and upper crankcase for inspection. It is not a practical installation for a fuel injected system.

The value and efficiency of the system is apparent when world record holder Jon Sharp depends on getting the most performance out of his Nemesis with a similar enclosed compression chamber set up.

Oil Leaks That Aren't

Try to keep your engine clean. Then, should an oil leak develop, it will be easier to detect and its source easier to locate. Pinpoint the location of a new oil leak as soon as it appears. A tiny oil seep can make a big mess. Unfortunately, these small leaks are difficult to trace to their source. Because of the diverse flow of cooling air through the engine compartment a bit of oil can appear as an oil slick far removed from its leak source. Sometimes carelessly pouring oil into the oil filler can result in a bit of oil running down the filler neck to who knows where. Later, that bit of oil falsely takes on the appearance of a possible oil leak.

The same thing can happen when changing the oil filter. If all spilled oil is not wiped clean from everything, it could catch your attention later and cause you to be overly concerned over a non-existent oil leak.

Mounting Small Components and Parts

The practice of supporting fairly heavy objects (fuel and oil pressure senders, exhaust tail pipes and air filter units, for example) by clamping them to the engine mount tubing is common . . . but not always advisable. A better and more reliable installation results when such components are supported on the firewall with individual brackets.

Elastic Stop Nuts

The use of elastic stop nuts (AN364 and AN365), in place of all metal high temperature stop nuts (AN363 and MS21042), in the engine compartment is another practice usually condoned but not considered to result in a proper installation. Certainly, high temperature all metal stop nuts should be used for the installation of exhaust pipe and heat shield supports.

The use of the correct exhaust manifold nuts is another mandatory consideration. Incidentally, Continental engines use the brass type nuts while the Lycomings use the steel nuts.

I’m sure you realize this is not an all-inclusive review of engine compartment installations but it is fairly representative as to what constitutes proper installation practices for them.

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