Matt's RV-7 Project

The ignition leads for the magneto all originate from a sort of combination cap/connector thingy, which screws into the back of the magneto housing:

It only goes on one way, so it's hard to mess up. I had to use a Torx bit and a ratchet to get the screws in. Also note how close the ignition leads are to the prop governor cable. This is another time where I wish I'd known then what I know now – I might have moved the firewall penetration for the prop governor an inch or so to keep it away from the mag. Remember what I said earlier about chickens and eggs?

The firing order of the magneto isn't specifically written on the unit anywhere, other than a symbol that identifies the lead for the #1 cylinder. I found what I hope is the correct firing order in the Lycoming overhaul manual. In my installation, the magneto fires an aircraft-type plug in the bottom hole of each cylinder, and the P-Mag fires a set of auto plugs on the top side.

The #1/#3 plug wires from the mag head over to the right side of the engine, and the #2/#4 leads from the P-Mag come back the other way, so there's a continuous bundle of plug wires strung across the back of the engine. High-temperature tie wraps hold it all together, and keep the wires out of the way of the prop cable.

Moving around the lower left side of the engine, the bundle of ignition leads ducks under an engine mount tube and turns the corner with the help of an adel clamp affixed to the bottom of the #4 cylinder baffle. You can also see where I put cardboard on the oil cooler to keep the delicate fins from being smashed by a wayward wrench.

Clamps on the rocker covers bring the ignition wires forward to the bottom plugs on #2 and #4. I used high-temperature silicone adel clamps here since they're screwed directly to the hot cylinder heads. Maybe not necessary, but I felt like doing it.

Aft of the #4 cylinder, the wire bundle splits in half and a pair of leads heads upwards, inboard of the oil cooler. You can sort of see where I used a pair of adel clamps to affix those two wires to the fitting on the bottom of the oil cooler.

Here's a view of the same pair of wires, looking down from the top. After passing up between the oil cooler and engine mount, they turn forward again and run through the special grommet in the rear baffle.

Moving back to the rear of the engine, here's another view of the "highway of ignition wires" that runs horizontally across. The P-Mag uses automotive style push-on ignition leads; the firing order is fixed but well-documented.

The wire bundle that goes to the #1/#3 cylinders snakes through the engine mount before splitting to go to the top and bottom plugs. The way this ended up working is kind of clever – the top plug wires are attached to the rear of the #3 cylinder baffle, which helps lift up the whole wire bundle and keep it away from the engine mount.

At the rear of the #3 cylinder, I used a pair of adel clamps on the end of the manifold pressure hose to guide the lower plug wires. I will probably put a drop of RTV in between the wires and the blue oil drain fitting in the center of the photo, since they come pretty close to it.

Same setup for the lower plug wires on this side – clamped to the valve covers and looped back onto the spark plugs:

Here's a view of how the upper ignition leads for the #1/#3 cylinders curl around the oil dipstick and penetrate the #3 cylinder baffle. I riveted on a doubler to reinforce the area where the adel clamp attaches; my 4" squeezer yoke was just long enough to do the job with the baffles installed on the engine. I may not have had to do this if I'd located the pass-through grommet a couple inches further outboard, but like a dummy I used the location shown in the plans and assumed it would be correct (ha!). It all worked out in the end, though.

For securing the ignition leads on top of the engine, I made a set of little angle brackets from 4130 steel, painted for rust prevention.

These I attached to the same clamps that brace the fuel injector lines to the pushrod shroud tubes:

Then I used yet more adel clamps to secure the plug wires as they fly over the top of the engine:

Same thing on the other side, although only a single clamp is needed due to the way the cylinders are offset:

It's all very neat and tidy on top of the engine now:

It took a lot of head scratching to get to this point, but in the end I'm happy with how the ignition harness routing worked out. All the wires are securely fastened in place, and nothing is in danger of rubbing on the engine mount, nor any other fixed structure, even when the engine shakes around.

I'm not sure if I ever mentioned that I switched my electronic ignition plans. Did I mention that? Anyway, I sold my Lightspeed ignition box after I got fed up with Klaus's idea of customer service, and bought a P-Mag instead. Based on what you read on the internet, they are either the greatest thing since sliced bread or certain doom. I'm not sure which version is closer to the truth. No product is perfect, but all the same I'm happy with my decision to keep one stone-age magneto for redundancy. Magnetos aren't foolproof either, but at least they have different failure modes, so maybe they won't both break in the same way at the same time (see also the Swiss cheese effect).

Having said all that, if the P-Mag ever so much as looks at me funny I'll replace it with another magneto so fast it'll make the propeller spin. Reliably, I hope, which is the whole point.

For reasons known only to Lycoming, the magneto drive gear is regarded as an engine component, not part of the magneto. So if you're installing a P-Mag on a new engine, you have to track down a gear to use with it. The correct part number is 61163; be prepared to shell out a hundred bucks or so for the privilege.

Remember to install the Woodruff key on the drive shaft before attaching the gear:

A strap wrench (aka oil filter wrench) is helpful for torquing the nut on the shaft, since there's nothing else to grab hold of and you don't want to squash your expensive gear in a vise. The cotter pin is fun to install – be sure not to leave anything poking out that will catch on the delicate engine-innards and ruin your day.

Once again, don't forget the gasket:

One nice thing about the P-Mag is that, unlike a traditional magneto, it's completely insensitive to mounting orientation. I put it on with the connector pointing outboard for easiest (or at least, least difficult) access. Sharp-eyed readers will notice that the oil pressure hose is disconnected in this photo, of necessity to allow access to the accessory case.

I got a start on running the low-voltage wires to the P-Mag (power, ground, tach, and key switch), but got stalled by lack of a suitable ring terminal to attach the ground wire to an engine case bolt. I'll get to it eventually.

Let me also say something about the electrical connectors they chose to use for these connections. In a word, they stink. In my opinion, this type of screw-terminal connector is acceptable for a piece of laboratory test equipment, but not very well suited to the harsh environment around an airplane engine. I assume they used these because they are cheap and convenient for the designer, if not the installer. I much prefer to have something a lot more robust like a cannon plug used here, and I pointed this out to the P-Mag guys the last time I saw them at an airshow. Their response was to tell me to go pound sand, which I thought was nice. In theory it should still keep running even if all the wires fall off, but then again I have also heard reports of problems happening if or when the ground wire breaks. I will probably end up crimping little ferrules onto the ends of my wires in an attempt to mitigate any future problems, following the lead of this VAF poster.

This is a magneto. It makes sparks to make the engine go. It's ancient technology but it works without any source of external power and it doesn't have a single bit of software in it.

The silver business between the magneto and the drive gear is an impulse coupler. It's basically a spring-loaded clutch that changes the timing of the magneto during starting. When the magneto is turning slow, the spark timing is retarded by a set number of degrees so the cylinders will fire when the piston is right about top dead center (TDC). When the magneto is turning fast, it uses the normal timing that you set when you installed it. The amount the timing changes is call the lag angle. (Protip: Never take one of these apart if you can help it. I watched an amateur try to repair one once; the big nasty coiled-up spring came un-coiled and threw little parts everywhere. Some were probably never found.)

Here's the data plate for the magneto. The lag angle stamped on it is 20°. The "L" stands for "left-handed rotation", not "mount on left side of engine". It means that, when viewed from the rear of the engine, the mag will rotate counter-clockwise (left) when the engine is running. Although, confusingly, we actually are going to mount it on the left side of the engine.

The thing sticking out of the back of the magneto is a T118, better known to you and me as a timing pin. You want to gently insert it into the hole that corresponds to the mag's rotation direction ("L" in this case) and wiggle the gear train around until you feel it drop in. Here I'm holding a second pin to give you an idea of what to look for. You want to have it drop in to where the first shoulder (the smaller-diameter one closest to the tip) hits the rear case of the mag. The much larger shoulder is just a red herring put there to confuse you into thinking that it has some function – ignore it. Oh, and if you have an impulse-coupled mag like I do, it's helpful to rotate the mag in the opposite direction from its normal rotation so you don't accidentally get the timing wrong due the impulse coupler engaging while you're fooling with it.

This is the data plate that's attached to the engine. Note that it specifies a spark advance of 25° before top dead center (BTDC). The magneto, however, has a lag angle of 20°, which means that if you time the mag to the engine at 25° BTDC, it will actually fire at 5° BTDC during starting, not right at TDC like you want. Is this a bad thing? I don't know – I read a half-dozen VAF threads on this subject, and none were very conclusive. Like most internet discussions about any subject, opinions ranged from "You'll be killed immediately" to "I've been doing it that way for twenty years and it's fine".

Luckily I don't have to choose for good just yet. I arbitrarily picked a timing of 25° BTDC for use with my 20°-lag-angle-having magneto, on the grounds that that's how it came to me from Mattituck (RIP). And also because I'll re-set and re-check the timing before I actually go to start the engine.

Anyway, what you do is remove all the plugs so the engine can be turned by hand, and rotate the crankshaft in the normal direction (clockwise when viewed from the cockpit) until the #1 cylinder reaches the compression stroke. Keep turning until the desired timing mark lines up with the hole in the starter. If you go past it, rewind several degrees and then come up on it from the normal direction, in order to take up any gear lash in the accessory drive.

Then you install the mag on the engine, with the timing pin still inserted. The exact angle of the mag with relation to the engine isn't critical, although in my case there is only a narrow range of positions where it will actually fit without hitting something else. What you want to do is gently insert the mag into the accessory case and get the gear teeth to line up, then put the nuts on finger-tight. Don't forget the gasket. Also don't rotate the mag or the crankshaft or you'll shear the timing pin. Once everything is in place, remove the timing pin and put it somewhere safe where you'll be sure to forget it later.

Then you fine-tune the magneto timing, using a magneto timing synchronizer, colloquially known as a "buzz box". Aircraft Spruce sells a wide variety, although they all do the same thing. I have a cheap low-end one that's so cheap it doesn't even buzz, but rather emits a terribly annoying smoke-alarm shriek. I taped over the speaker to mute it a bit. You clip the black lead to ground, the green lead to the P-lead terminal on the left mag, and the red lead to the P-lead terminal on the right mag (not present on my airplane). Then you slowly rotate the mag back and forth until the points are just opening at the desired spot. Tighten the mounting nuts, then rock the crankshaft forwards and back and verify the timing is set how you want it. Repeat annually to check for timing drift due to wear.

The above represents my interpretation of how to install a magneto, based on what I could piece together from web searches, printed material, and consultation with the village elders. Your mileage may vary – consult the instructions that I hope you got with your timing box. This document is also a good starting point.