Piper J3
Well Known Member
This is weird....
Three years ago, I was diagnosing a rough running engine and replaced all eight spark plug caps thinking, at the time, maybe the 5 ohm resistor in one or more of the caps was damaged. The rubber boot gets manhandled when pulled from the spark plug and my thinking was the internal resister could get damaged. I replaced the rubber caps with NGK 90 Degree Elbow (LB05F) 8051. This spark plug cap is rigid phenolic and my thinking was the internal resistor wouldn’t get damaged easily.
Engine has been happy for over 300 hours until today. Flew in a lot of turbulence this morning and engine began running rough. I shut off one ignition at a time in flight and it showed both ignitions working, but one very rough. Returned to home base and removed top cowling to find top left rear spark plug wire not attached to the spark plug.
I pushed the sparkplug cap back on with no ratcheting sound and cap pulled right off. There is a spring circlip type wire in the cap that engages with the exposed screw thread on the sparkplug. When pushing the cap on the circlip wire “ratchets” down the sparkplug threads and becomes engaged in the trough of the last thread to hold the cap in place. I thought this was a very well proven design.
So, what happened? The circlip wire apparently is harder than the threads on the sparkplug and vibration most have caused the wire to wear a flat against all the exposed threads allowing the cap to lose its grip and come off. See attached photo...
Now I’m researching the correct NGK rubber sparkplug cap for 912ULS. I think the rubber cap does two things – prevents water ingress, and also holds the cap tightly to the porcelain of the sparkplug to prevent movement.
It’s always something Gilda….
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Three years ago, I was diagnosing a rough running engine and replaced all eight spark plug caps thinking, at the time, maybe the 5 ohm resistor in one or more of the caps was damaged. The rubber boot gets manhandled when pulled from the spark plug and my thinking was the internal resister could get damaged. I replaced the rubber caps with NGK 90 Degree Elbow (LB05F) 8051. This spark plug cap is rigid phenolic and my thinking was the internal resistor wouldn’t get damaged easily.
Engine has been happy for over 300 hours until today. Flew in a lot of turbulence this morning and engine began running rough. I shut off one ignition at a time in flight and it showed both ignitions working, but one very rough. Returned to home base and removed top cowling to find top left rear spark plug wire not attached to the spark plug.
I pushed the sparkplug cap back on with no ratcheting sound and cap pulled right off. There is a spring circlip type wire in the cap that engages with the exposed screw thread on the sparkplug. When pushing the cap on the circlip wire “ratchets” down the sparkplug threads and becomes engaged in the trough of the last thread to hold the cap in place. I thought this was a very well proven design.
So, what happened? The circlip wire apparently is harder than the threads on the sparkplug and vibration most have caused the wire to wear a flat against all the exposed threads allowing the cap to lose its grip and come off. See attached photo...
Now I’m researching the correct NGK rubber sparkplug cap for 912ULS. I think the rubber cap does two things – prevents water ingress, and also holds the cap tightly to the porcelain of the sparkplug to prevent movement.
It’s always something Gilda….
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