By now, Kitplanes readers should be pretty familiar with magneto-alternative electronic ignitions. Especially the models made by E-MAG, as Myron Nelson, Vic Syracuse and Paul Dye have written articles about their products. Another is the SDS (Simple Digital Systems) CPI system, which, together with their electronic fuel injection, is about as close as you can get to a modern automobile for integrated engine management. That also has been written about in the pages of Kitplanes. Mike and Laura Starkey have an SDS system on the IO-320 Lycoming in their RANS S-21. (Mike and Laura’s 16-part build series ran from 2020 to 2022. To read about their SDS installation, see the April 2021 issue.)
Among other advantages, such as stronger spark, better timing control, lower maintenance and longer inspection intervals, these ignitions allow the use of auto spark plugs. Auto plugs are desirable mainly due to the cost, which can range from $2 to $10 each. Compared to aviation plugs, which cost $40 to $160 each, that’s a bargain, especially when you have to replace eight or 12 plugs. In case you’re wondering if auto plugs sacrifice reliability, it has been mentioned that virtually all “new design” aircraft engines—Rotax, UL, Jabiru, MWFly, etc.—use auto plugs.
Switching to auto plugs in Continental or Lycoming engines commonly involves using 18mm-to-14mm spark plug adapters. A quirk of installing these adapters is that you torque the adapter to the cylinder head with the plug installed. In the case of E-MAG, their instructions are specific: “FIRST install each spark plug in the adapter (fully seated and finger tight). THEN, insert the combined plug/adapter assembly in the engine and tighten to 18 ft/lb (standard auto plug torque) through the spark plug ONLY. Do NOT torque the adapter itself.”
Both Paul Dye and Vic Syracuse have talked about this. The main issue seems to be if you torque the adapters into the head without a plug, it could distort the thread or otherwise damage the adapter. Once installed, you can usually remove and replace the plugs as normal.
Done correctly, plug adapters do the job. Occasionally—maybe one in 10, according to Paul—when removing a plug for a compression check or routine replacement, the adapter will unthread with the spark plug.
In order to prevent that annoyance, Mike Starkey came up with the idea of a holder wrench that would provide a way to reliably remove spark plugs without unthreading the adapter. It’s important to note that Mike’s wrench is for holding only, not for tightening or loosening.
Most brilliant ideas are simple and this is as good an example of that as any. Simply cut the drive end off a ⅞-inch 12-point hex socket and machine the remaining ring just long enough to engage the hex on the adapter. To that, weld an appropriately designed handle. What you end up with is a special box wrench that fits into the very close-fitting margins of the plug counterbore. Hold the handle of the adapter holder with one hand, and using a plug wrench, loosen the spark plug with the other.
From a machining standpoint, it’s relatively simple, except for the fact that most mechanic’s sockets are heat-treated, tempered and chrome-plated. It is therefore critical to use a carbide insert turning tool to remove the chrome. The main reason to remove the chrome is because chromium fumes can be nasty when welding. A secondary reason is, while the underlying steel socket might be heat-treated to somewhere between 35 and 50 on the Rockwell C hardness scale (HRC), the chrome plate could be as high as 70 to 75 HRC, which makes it tough to saw through.
That’s it for now. Time to get back in the shop and make some chips!
