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:
