SonexBuilders.net

Today as I was preparing to inspect the cylinders and heads of my Jabiru engine I removed the spark plugs and found something very unexpected. Three of the plugs on the pilot-side bank showed signs of leaking, and were very lose when removing them. The other bank plugs were all fine. Each of the lose plugs were a rear plug, and I suspect that when I had those plugs out last several months ago I did not torque them properly. I'd like to think that I would never have made such a simple mistake as not torquing them properly, but it's either that or the plugs came lose on their own.

I've included a picture link to the worst offender. You can clearly see that the right hand plug in the photo (the rear plug) show exhaust leakage past the threads and deposits on the plug body. I didn't notice any unusual vibration, excessive CHTs or loss of power - strange to think that the engine ran pretty good despite the leak.

http://sonex604.com/misc/plug.html

My take-aways here are: 1) double-check your own work when doing maintenance, 2) regular maintenance and inspection can help find small problems before they become big problems, and 3) a dose of humble pie every so often keeps you on your toes!

Y'all be careful out there!

Jeff

It appears as though the gasket on the offending plug is not compressed as much as the other, which would indicate a faulty torque and not a problem with the plugs or engine.

Jeff,

Did you notice some time back that Jabiru changed the torque value on spark plugs from 8 ft-lbs to 12 to 14 ft-lbs? This would be easy to overlook. What were you using?

Art,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

I did the same thing on a 2200 Jab powered Sonerai years ago. I could hear a different sound and noticed the performance difference, and aborted the takeoff. The extra power of the 6 cyl would no doubt make it harder to notice.