Matt's RV-7 Project

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.

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: