Bad Spark Plug

[pilotyoung]

I had an unusual experience today when flying my Onex. I did all the checks before takeoff including a runup and everything was ok. I took off and flew around for about 30 minutes and decided to turn off the primary ignition to see if the CHT's changed to check the timing. Almost as soon as I turned it off, I noticed that I did not have any EGT on the number 3 cylinder. So I turned the primary ignition back on and headed straight back to the airport and landed. When I got to the hangar I took the top cowling off and noticed the ignition wire for number 3 cylinder was laying on the bottom cowling. As I inspected it I realized that the screw on tip of the spark plug had come off the tip off the plug and was still in the electrical connector. So I took it out and then took the spark plug out. The ceramic part of the spark plug would spin inside the metal base that screws into the cylinder. I presume the ceramic part was spinning due to vibration and that is why the tip came off. I have never seen a spark plug fail like this.

Has anyone else experienced this?

Thanks

[n502pd]

hello! yes, I have seen what you describe, but maybe not to the extent that you indicate. I have seen a fair number of plugs exhibit dark stains, from a little bit, to covering the porclin insulator up to where the boot is. This is due, IMHO,to the crimp holding the insulator, and its internal gasket (if there is one on the brand you use), to the metal body, being not fully crimped, and isnt tight, hence gasses leak out and cause the deposites. Those plugs are defective and should not be reused, except in a leaf blower, or sling-shot!. when i instal new plugs, I take a pliers to the nut on the plug top and carefully tighten it, as they are mostly quite loose out of the box, if not already off. Be careful, in years past I have fount one brand that, when tightening the nut, the whole top of the insulator electrical connection comes off with very little effort. New plugs, at least for my AeroVee, are not expensive, considering potential failures described. Fly safe!

[pilotyoung]

I looked at the plug and it does have a brown stain around the porcilin. Here is a picture of it.

I use Autolite 4163 and put in 8 new plugs each year during the condition inspection. I always tighten the cap with pliers also. The current inspection expires at the end of November so it is do for new plugs in a week.

Thanks for your comments.

[n502pd]

I use the same plugs, and sometimes see a couple with that ammount of stain! Plugs are indeed cheap. I carry a set of 4 with me in the plane!