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Engine Hook Up Tips

By Tony Bingelis (originally published in EAA Sport Aviation, August 1989)

As pointed out last month, most of your bolt-on engine accessories and parts should have been installed before hanging the engine. Access, now that the engine is bolted to the mount and firewall, will be quite restricted.

Check to see how much clearance you actually have between the engine accessories and the firewall. For example, check the space between the firewall and the magnetos. Also, if a propeller governor is installed, check the space between it and the firewall.

If there isn't much room there, you will have to accept the fact that sometime in the future the engine will have to be pulled off its mount before the magnetos can be removed. But worse than that, you may find that you will have to cut a recess into the firewall to provide the space needed for the installation of the ignition harness. The same problem may affect the propeller governor control mechanism when a governor is mounted on the rear accessory case.

If the firewall clearances present no problem, you are ready to begin hooking up and installing the works. The big items will include the ignition system, fuel and oil systems, exhaust system, engine controls, wiring, and the installation of fittings and connections for the engine monitoring instruments ... to mention a few of the more important tasks ahead.

Obviously, covering all these subjects - and more - in detail would be impossible in the short space available here. Actually, I could write a book on the individual subjects . . . as a matter of fact, I did. My innate modesty not withstanding, I seriously recommend that, if you have yet to install an engine in your project, you acquire a copy of my book, "FIREWALL FORWARD". It contains much more information on engine installations for homebuilts. (Order from EAA - 1-800/843-3612.)

Well, that may be O. K. for later, but just where would be a good place for you to begin right now?

I guess there really is no single best starting place for making the engine hook-ups. Much, of course, will depend on the complexity of your aircraft.

However, even a simple engine installation with no electrical system will entail plenty of work by the time you complete the fuel connections to the carburetor, hook up the throttle and carburetor controls, and connect the ignition harness correctly. Naturally, all engine installations, even simple ones, must be fitted with the mandatory minimum VFR instrumentation listed in the Federal Aviation Regulations (FAR Part 91). Included in that list are the three engine monitoring instruments, namely, the oil pressure gauge, the oil temperature gauge (a water temperature gauge for liquid cooled engines), and the tachometer. Let's start with the most logical things to be done first.

Establishing the Aircraft Ground

Without a doubt, your first engine hook-up should be the grounding of the engine to the engine mount, and to the aircraft. Remember, your engine is insulated (electrically) from the engine mount and the rest of the aircraft by rubber shocks.

An excellent aircraft electrical ground is virtually guaranteed by connecting a short, fairly heavy, flexible grounding strap (or a stranded AWG No. 8 cable fitted with terminal connectors) between the engine and the engine mount.

Careful . . . the ordinary fine wire low capacity electrical bonding jumper straps generally used for aircraft bonding are not heavy enough and should not be used for this purpose.

The aircraft grounding connections should always be made with nuts and bolts . . . never with rivets.

Secure one end of the ground strap to most any nearby engine bolt, and the other terminal to a welded tab on the engine mount or some other solid point.

NOTE: Never install any kind of bracket or aluminum connector under any of the engine's cylinder base hold-down nuts.

Some builders like to install two grounding straps - one on each side of the engine.

In wood and composite aircraft, a special effort must be made to ground the instrument panel to provide a convenient aircraft ground for the wiring to be completed inside the aircraft.

If your airplane has an aft mounted battery and the aircraft frame is wood or composite, you should run the battery ground cable (same size as the power cable) all the way forward through the firewall and connect it directly to the engine. Normally, a No. 4 insulated stranded copper cable is heavy enough for most homebuilts. A lot of builders use a stranded No. 4 electric welder cable because it is priced right and is readily available at most welding supply sources. Commercially produced aircraft often use a heavier No. 1 cable.

Next, The Ignition System

Unless your magnetos are of the internal grounding type, they will be hot until you hook up the "P" leads (ground wires).

Although you may not yet have fuel in the tanks, play it safe. Do not install the propeller until the magnetos are properly connected and grounded by an ignition switch (Figure 1).

The magneto circuit is simple. All you need to do is run a single wire from each magneto to an ignition switch of some sort . . . even a toggle switch, if you prefer. The wire size requirement is small and a No. 18 shielded wire is adequate.

If no radio is to be on board, the magneto ground wire ("O" lead) need not be a shielded wire. However, the way the FAA is cranking out new governmental mandates, I would even be tempted to install shielded magneto wires in an aerial "putt-putt" - just in case.

You might also consider installing an individual magneto filter on each magneto to eliminate ignition noise. On the other hand, expending about $35 for two magneto filters might not be necessary. The shielded spark plugs and ignition harness, backed up by the shielded magneto "P" leads, may be all you need for noise-free radio reception. Anyway, you can always retrofit the filters later as they simply connect to the "P" leads at the magnetos.

Before you go any further . . . if you haven't already installed the engine cooling baffles, maybe you had better take on that job next. One reason for this is that your ignition harness will have to be routed through the back baffles. Furthermore, the proposed location for the oil cooler may also require the presence of the engine cooling baffles before it can be installed.

Some Thoughts About Engine Controls

You can proceed with your other engine hook-ups in any sequence you find convenient.

I like to work from the inside areas out. That is, I generally find it easier to install the engine controls to the carburetor before access there becomes more difficult with the installation of the exhaust pipes, heat muff, mufflers, fuel and oil cooler lines, ducting, etc.

Here is something else to consider. Your planned hook-up sequencing may have to be altered when you find that you don't have all the controls, hose fittings, wires, nuts and bolts needed to complete a particular installation. This happens more often than not. But don't let that deter you. Simply proceed to some other hook-up you can complete working with the parts you already have on hand. After all, everything will have to be installed before you operate the engine.

Unless you have a computer memory, it would be a good idea to start a "need-to-get list" so you won't forget to order those hardware parts you didn't know you would need.

In the interest of safety, discipline yourself to complete securing both ends of each assembly and each circuit before leaving it for some other task. Believe me, it is so easy to overlook replacing that incorrect nut, or safetying that part sometime later. The "I'll get back to that later" mentality is a bum habit to develop.

What kind of throttle do you want? Decide whether you want a fighter-type left side throttle quadrant, or would be willing to settle for a simple push-pull throttle knob with a thumb operated knurled friction lock. Your parts requirements are different for each type.

In a side-by-side two seater, the preferred throttle location would be in the vicinity of the lower panel center. In that location it would also be accessible from the right seat.

Installing a lever type throttle will always result in a heavier, more complex, installation as you will have to fabricate a bellcrank or two and possibly have to devise and install a torque tube assembly on the firewall. Usually, this is necessary to reverse the throttle lever action so that shoving the throttle lever full forward will cause the carburetor throttle arm to move to the wide open position. Another problem associated with lever type engine controls is the need to obtain the correct length flexible controls with threaded ends. I do not recommend using a throttle control that terminates in a bent wire arrangement - no matter how it is secured.

And I certainly do not recommend using a vernier type throttle knob. They are ideal for the mixture control and prop control, but entail too great a risk when using one for the throttle. The risk? You might forget to mash the center button when you suddenly decide you need full power. That vernier throttle won't budge unless you first depress that center button. Forgetting to do so in an emergency situation is very possible . . . and very dangerous.

Since the carburetor is situated under the engine on the aircraft's centerline, you should have saved a clear area in the lower central portion of the firewall to afford the most direct routing for the throttle and mixture control.

Unfortunately, it is most unlikely that you will find a convenient nearby component or bracket to which you can clamp the throttle and mixture control shaft housings. Since this problem also effects the installation of the propeller governor control, you may be forced to construct a special bracket for it also.

Although most builders ultimately manage to secure their engine control housings to a fairly rigid bracket, and at the proper distance from the carburetor levers, sometimes an otherwise good installation is jeopardized by using rubber cushioned clamps to immobilize the throttle and mixture control housings.

This is a poor way to do it because the cushioned material is not generally fuel resistant, and when it becomes oil soaked (and it will) may allow the control housing to slip. Just think how embarrassing it would be to push in the throttle and have nothing happen.

Always use metal to metal clamping to immobilize your controls and bowden cable housings at the appropriate distance from the carburetor levers. It is the safer way to do it.

Make sure the carburetor lever moves to the wide open position when the throttle is pushed forward.

The mixture control, likewise, must cause the carburetor lever to move to the rich position when moved forward.

Both controls - actually, all engine controls - should have a little bit of spring-back when moved to the full forward position in order to ensure getting maximum travel and output.

The positive way to verify this is by having someone move the controls in the cockpit, at your command, while You look for the proper movement at the engine. There should be no binding or rubbing throughout the travel limits of any engine control . . . nor should there be the slightest flexing at the point where the flexible control housings are clamped.

Don't forget to make provisions for carburetor heat. Use the red (SCAT) high temperature resistant ducting to conduct the hot air from the heat muff to the carburetor. Although all carburetor equipped engines must have carburetor heat, fuel injected engines seldom have a like arrangement or requirement.

One more reminder, try to obtain the correct length for each of the flexible engine controls. Extra long lengths are difficult to accommodate without introducing an excessive number of bends and increased friction in the control's operation.

The All Important Fuel System

Know what type of fitting is required for each assembly. For example, you cannot safely screw a straight threaded flared fitting into a pipe thread.

Although most components for light aircraft are tapped for a 1/8" pipe thread, don't assume this is always so. There are exceptions. The later style engine driven fuel pumps now have straight threads which require the installation of an "O" ring with the fitting so that it will seal properly.

The rule . . . check it out and be positive that the fitting you are trying to screw in is the correct one. A fuel leak can result in an engine failure or fire.

Be sure to use the correct 37 degree flaring tool to fabricate your aluminum lines. The automotive 45 degree flaring tool must not be used for aircraft fittings.

The minimum recommended diameter for an aluminum fuel line (5052-0 aluminum preferred) is 3/8" O.D. with a .035" wall thickness. For a flexible (rubber) fuel line this translates to a -6 size which has a 3/8" inside diameter.

The rigid aluminum lines may be used anywhere inside the aircraft up to the firewall. From the firewall to the engine, however, all lines must be fabricated with flexible hoses.

Because the engine is hung on shock mounts, there will always be a lot of movement between it and the airframe. Obviously, a rigid fuel line would soon break.

Carefully route your fuel lines so that there are no low places in them between connections. That is, try to run each fuel line in a constantly rising, or descending, slope.

1. Between the fuel tank and the fuel selector valve.
2. Between the fuel selector and the gascolator.
3. Between the gascolator and the electric fuel pump.
4. Between the electric aux pump and the engine driven fuel pump or
5. Between the engine pump and the carburetor.

Do you get the idea that this is an important factor?

Keep your fuel lines away from the hot exhaust pipes.

It is difficult, sometimes, to find the correct type fittings - especially when trying to plumb a paralleled fuel pump system (Figure 3). I notice that in some Pipers a heavy plain of grey iron "T" fitting is used at the carburetor. It looks like it might have come straight from the plumbing section of a hardware store . . . ‘nuff said.

The Electrical Staff
Select a wiring diagram you like and follow it.

The major things to remember are to use cables large enough for the loads expected, and to be sure to install shielded wires wherever needed.

Here's an example. The starter is a heavy current user and the cable connecting it to the starter solenoid should be at least a No. 4, possibly a No. 1, size cable. It need not be shielded as the starter circuit is inoperative in flight and will pose no radio interference problem.

On the other hand, the wires from the alternator do need to be shielded cables. A No. 12 or even a No. 14 shielded cable from the alternator "F" terminal to the voltage regulator "F" terminal should suffice. A heavier shielded cable (No. 10) should be installed between the alternator "B" terminal and the main engine circuit breaker at the bus bar.

At best, the typical engine compartment is a confused mess to view - makes me wonder if anybody could ever make that heterogeneous agglomeration look neat and orderly.

To help reduce this "offensive to the eye" engine compartment clutter, try bundling the wiring wherever practical. This means you may have to resist that weight saving impulse to make your wires as short as possible because that will only make your bundling efforts rather ineffective.

A Few More Observations

The Lycomings have a 3/4" oil breather outlet while the small 4 cylinder Continentals are fitted with a 5/8" outlet. In either case, use the correct size hose and do not attempt to step down the I. D. to a smaller size.

It's very much the same thing with the oil coolers installed on Lycomings. Use the correct fittings to accommodate the preferred -8 size hose.

And, finally, I would like to emphasize that a new homebuilt is entitled to new oil and fuel hoses even if you have to assemble them yourself.

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