Archive for the ‘Ignition’ Category

Right magneto

Sunday, November 16th, 2014

Several months after I'd spent hours securing the ignition harnesses to the engine and carefully routing them around a myriad of obstacles, I came across this discussion warning of the dangers of bundling P-mag ignition leads close together. There actually is a cryptic mention of this in the P-mag installation manual, but the way it's worded made it sound like it didn't apply to my installation. To check the information in the thread I emailed Emagair, who confirmed it but were also frustratingly less specific than I'd have liked when I asked for guidance on how they really wanted me to route the wires.

You may recall when I said "if the P-Mag ever so much as looks at me funny I'll replace it with another magneto". Rinky-dink connector, a history of failures, and now this? Three strikes and it's out of there. I'm tired of messing around with it and wondering if it's going to come back to bite me later. That's just my opinion, and I know plenty of people have used them on their airplanes successfully, but I'm done. Magnetos might be old-fashioned and prone to certain types of failure, but at least they're well-understood, thoroughly documented, pretty reliable, and easy to repair.

I traded the P-mag to somebody who was more eager to experiment than I am, and received a pair of used Slick 4370 magnetos in return. One I put on the shelf for spare parts, and the other I sent to Crossfire Magneto Service for overhaul. Meanwhile, I also bought a new harness (Champion P/N M2992R). That arrived at about the same time magneto came back, now looking brand new.

P-mag removed, good old fashioned magneto mounted in its place. The ignition harness is only loosely routed in this photo:

I used my magneto timing box (visible at right) to get both mags timed to 25° BTDC. It took some fiddling to get them in perfect sync, as it's hard to make very precise timing changes without a certain amount of trial and error. But I got there eventually.

Ignition harness routed and secured properly once again. The plug wiring is now a bit different from before – now each magneto fires the two bottom plugs on one side of the engine and the two top plugs on the other side. The bottom plugs always suffer more oil fouling than the top ones, so it makes sense to do it this way. It seems like this would also tend to equalize the RPM drop from one side to another during a pre-flight ignition check.

The plug wire routing on top of the engine is slightly different from before due to the plugs being taller and having straight as opposed to right-angle connectors.

The P-lead routing for the right mag was kind of a pain – mostly due to all the adel clamps I had to put in to secure this one wire.

I used the same arrangement for strain-relief of the P-lead as I did on the other side – an adel clamp attached to the ground lug with a slightly longer screw and lockwasher. Wiring the right P-lead was made more difficult by the fact that you have to attach the fasteners from the bottom in an area you can barely reach, thus giving the little nuts and washers all kinds of opportunities to fall onto the floor and go skittering off to who knows where. I eventually got it sorted though.

And with that taken care of, it's back to forward progress I hope…

Magneto P-lead / RPM sensor / oil temp probe

Sunday, December 29th, 2013

Three projects in one post today, for reasons that will become clear…

First, I fabricated the business end of the magneto P-lead. This connects to a switch in the cockpit that operates in reverse of the normal fashion. When the switch is open, the magneto can operate and generate its own sparks. When the switch is closed, the magneto is grounded and can't fire. For historical reasons unknown to me, the terminals on the magneto to which you connect this pair of wires are called "GND" and "P", hence "P-lead".

The open hole partially visible in the center of this image is where the oil temperature sensor goes. The hex-shaped thing below it is the exterior portion of the Vernatherm, which is a fancy trade name for "thermostat". The stuff at the top of this picture is where the oil filter threads on. Sorry for the terrible photo, this area is really close to the firewall so I can barely get my camera back there, let alone see it when I'm trying to work on it.

This is what the oil temp probe looks like. I crimped on a connector so I can replace it more easily in the future if I have to.

Oil temp probe installed with a new AN900 copper crush gasket. Crush gaskets are kind of interesting since you tighten them by turning to a specified angle rather than using a torque wrench. Since the hole uses a 5/8-18 thread, this gets tightened to 135° beyond finger-tight.

To install the RPM sender on the magneto, I had to remove the mag from the engine. Ugh! Wish I'd thought ahead a little more. Oh well.

The RPM sender threads into an unused vent hole on the mag, which is usually capped with a hex-shaped plug.

RPM sender installed in mag, using blue loctite and tightened to the specified torque. The P-lead and ground screw hole are visible at left.

Mag reinstalled… I used the timing pin and attempted to get it timed properly, although I will revisit the timing again for both the magneto and P-mag at some point in the future.

Now here is why I'm covering three otherwise unrelated wiring projects in the same post. All of these wires – P-lead, oil temp, and RPM – pass through the same area on the back of the engine, and all of them need to be supported so they don't vibrate and eventually break. The most convenient place to anchor these wires is actually the hole for the P-lead ground screw itself.

So what you're looking at here is the back of the magneto, with an adel clamp affixed to the hole that normally just gets a ground terminal; the ring terminal for the ground lead is between the screw head and the clamp. The other P-lead conductor loops around and attaches to the magneto "P" terminal, hidden under a rubber boot. Meanwhile, the wires for the oil temperature probe and the RPM sender pass through from left to right and are restrained by the clamp.

Again, sorry for the photo quality… this area is hard to see, hard to work on, and hard to photograph.

Moving upwards, the wire bundle loops around an oil hose and then attaches to the engine mount. Where there is currently masking tape holding things temporarily, I will eventually put a pair of adel clamps to grab onto this wire bundle. As with the bus current sensors, I used silicone tape around the connector for the oil temp sensor to keep water out.

Same area, this time with the oil filter installed:

Another glob of silicone tape to protect the connector for the RPM sensor, and then all three sets of wires pass through the firewall:

By the way, I did not think of this clever wiring trick myself – I got it from this VAF post.

P-mag manifold pressure hose / battery fit check

Sunday, December 15th, 2013

The P-mag requires a manifold pressure connection in order to control its timing advance. I had previously located a tee fitting in a convenient location, but I never finished it since I didn't have the right fittings on hand. Time to check that off the list, so here's a combination of fittings that allows me to tap off the 1/4" OD plastic hose with a 1/8" ID rubber line to the P-mag:

Here it is mounted on its little bracket. Note the pair of adel clamps on the left side of the image – they will become relevant later.

Manifold pressure line connected to the P-mag. I used the stuff that came with the P-mag, but it seems like it might just be regular old vacuum hose from the auto store.

Since I was working with the P-mag, I plugged in the power supply and verified that it gets power when the master switch is on (note green LED). I also set the crankshaft to TDC and blew in the tube to set the initial ignition timing, although I will probably adjust it again before I try to start the engine.

At some point while I was messing around in this area of the firewall, I started thinking, "hmm, there sure are a lot of things running through this area, wouldn't it suck if it turned out to be impossible to install and remove the battery now?" So I dug the battery out of the basement and tried a test fit. It's really tight with all that stuff in there! I had to push the ignition wires, ground strap, and oil pressure hose out of the way…

Almost thereā€¦ it's like getting a battery-sized square peg through a series of triangular and oblong holes…

Home free now! From here it just drops straight down into its tray. Of course then I had to reverse directions and remove the battery again, which was equally painful.

Here's that manifold pressure plumbing again. I had to remove the adel clamps in order to be able to flex the oil pressure hose out of the way of the battery. Subsequently I decided to just leave them off – if these two lines turn out to rub together, I'll fix it with a dab of RTV.

Kids, do yourself a favor and think about battery replacement when you're locating stuff on the firewall. I didn't give it enough forethought and I almost screwed myself. Lesson learned.

Top spark plugs

Saturday, December 14th, 2013

Today I installed the top spark plugs on all four cylinders. For posterity, here's a photo of the box of plugs I used… NGK BR8EIX iridium plugs, as recommended in the P-Mag install manual. The 6747 part number has solid terminals, not the two-piece screw-on variety, which is preferred for this application.

The back of the box has a "not for airplanes" pictogram. That's encouraging.

The engine came with a set of cheap Denso copper plugs. I'm sure these would work fine, but I thought I'd go with iridium plugs based on past experience with an airplane that had a car engine in it. Also I've been really impressed with how long the OEM iridium plugs in my car have been going, with no appreciable wear.

View of the old and new plugs showing the difference between the two types of electrode.

You're not supposed to gap iridium plugs by hand, but I did check the plug gaps just to make sure they were manufactured correctly. The manual recommends a gap of between 0.030" and 0.035"… all four of my plugs were 0.028", which I'll consider close enough.

To use auto plugs in an aircraft cylinder, you need adapters to make the 14mm auto plugs for the 18mm aircraft plug bosses. When I sold my Lightspeed system, I kept the Lightspeed steel adapters rather than buying new ones from the P-mag people. That just means that I had to refer to the LSE documentation to install the plugs.

Using the LSE instructions, first the adapters get installed and torqued to 35-40 ft-lbs, using anti-seize and a new copper gasket:

Then the plugs are installed in the adapters, again with anti-seize, and torqued to 18-20 ft-lbs. This is different from the instructions for the P-mag adapters, which have you thread the plugs into the adapters and then torque them into the heads all as a single unit (to 18 ft-lbs). Anyway, since I have LSE adapters I decided to stick with the LSE instructions.

I put a light coat of silicone grease on the outside of the plugs to help the boots seat properly, and pressed the boots onto the plugs until they clicked:

I also re-did the ignition wire support arrangement a bit… I had previously piggybacked the support brackets onto the existing adel clamps that secure the fuel injection lines, but a VAF thread made me consider that such an arrangement could lead to increased vibration and potentially even cracking of the stainless lines. Probably a remote possibility, but just to make myself feel better I put in an extra set of adel clamps for the ignition wire supports, and restored the fuel injection line clamps to their previous configuration. Now there's no connection between the fuel lines and the plug wire supports.

Meanwhile, I haven't final-installed and torqued the bottom plugs yet, so used some red tape to remind myself they they are only temporarily installed:


P-mag wiring

Saturday, September 21st, 2013

Time to run wires through this mess of engine mount structure and over to the P-mag:

An adel clamp on a handy screw provides strain relief for the incoming wires. And here we are looking at that obnoxious non-aircraft connector again.

I bought a cheap wire ferrule crimping kit, as recommended by certain folks who've been down this road before me. The ferrules are basically just little tubes which get swaged onto the ends of the wires.

This is what a wire looks like after having a ferrule crimped on, and then adhesive heatshrink tube applied as a further strain relief. Now at least there aren't any bare wire strands to get broken by that terrible connector.

Wires go along the upper engine mount tube, then loop down to the P-mag:

Hard to get photos of this area, too much stuff in the way:

Oil pressure hose reinstalled:

I actually didn't get around to testing these connections yet – I will eventually put 12V power on the airplane and see if the LED on the P-mag lights up.