Using 10mm spark plug.

Hi everyone.

I am doing an ?lectronic ignition upgrade on my aircraft and was not satisfied with the 18mm to 14mm plug adapters availlable.

They are too thin to torque properly and almost impossible to intall with a temp probe under.

My solution is to use 10mm thread plug, same NGK number but starting with a "C".

Pic of the problem:



Pics of the solution :















Negative input very welcome before I try to fly with this !!

If your new adapter is steel, that could be a poor choice due to relatively poor thermal conductivity compared to brass. That may alter the effective heat range of the spark plug.

Most engines are using deep well type CHT thermocouples as it's been found than under-plug types don't give an accurate picture of what's really happening. People flying Jabiru engines found this out by experimentation with both types.

18mm plugs or adapters call for about 25-30 ft lbs. torque which should be no problem with properly machined adapters made out of proper quality brass stock (not el cheapo lead/zinc alloy). 14mm plugs call for 15 to 20 ft. lbs. torque. I'm not aware of any widespread problems with QUALITY adapters.

Torque the plugs

Frank from Quebec said:

Hi everyone.

I am doing an ?lectronic ignition upgrade on my aircraft and was not satisfied with the 18mm to 14mm plug adapters availlable.

They are too thin to torque properly and almost impossible to intall with a temp probe under.


Negative input very welcome before I try to fly with this !!

Click to expand...

I am flying dual P-mags with 14mm adapters. The plugs and adapters are torqued together. In other words, the wrench only touches the spark plugs.

I like your plan. If the steel adapter may result in a hotter running plug (undetermined) then I suggest you start with a cooler spark plug than you originally intended. A cool plug that fouls is safer than a hot plug that supports detonation. Install your adapter with a thin coat of Nickel anti-seize on the threads.
Of course, you will be removing the plug often during the start of the test, so a cool plug would be dark and sooty. A hot plug would be white and a real hot plug might have heat damage to the tip. Perhaps you would test only one plug, or only the plugs on one mag so you could switch it off if you have a problem in flight.
I have been running 18mm Autolite 386 plugs in a low compression O-290. These run white (as top plugs) so I've been searching for an 18mm plug in a cooler heat range. I'm also looking for a projected tip plug. These are hard to find in an 18mm resistor plug. So I bought NGK resistor caps, with 5K Ohm resistors. Unfortunately, these require 7mm plug leads, and I'm running 8.5mm MSD super conductor wires. I'll need to drill out the plastic housing to adapt or make a new harness with 7mm wire.
I accidentally ordered some 12mm spark plugs with long thread reach, turns out they are used on Maserati cars and popular Japanese motorcycles. I was thinking the same as you about adapters because the latest research in spark plugs is going into these sizes.
I would like a nice resistor plug with projected tip that won't hit the piston or valves, but improve combustion ignition and resist fouling.

I agree about possible heat risk and will start with cr10e in every hole wich should be cool enough and then experiment with one at a time going slighly hotter and or projected tip like a cpr9e maybe iridium as any of them are availlable in eix version.

Not ready to fly yet but I will tell you how it goes when I do.