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  #1  
Old 09-26-2021, 06:50 PM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 2,567
Default Spark Plug Wire Fell Off In Flight…

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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Jim Stricker - EAA #499867
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80 hrs Flying Aeronca Chief 11AC N86203
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Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 750

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  #2  
Old 09-26-2021, 07:24 PM
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Tony_T Tony_T is offline
 
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Location: Lacey, WA
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“I thought this was a well proven design”
Everything on the 912 ULS motor is a well proven design. When you change something, there are almost always unexpected consequences.
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  #3  
Old 09-27-2021, 10:50 AM
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Piper J3 Piper J3 is offline
 
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So, I’m trying to find exact Sparkplug Cap for 912ULS. It appears that Rotax uses NGK brand but assigns an internal PN.

From what I see the cap is 120 degree angle rubber boot. What I don’t know is how it attaches to the sparkplug. Option is for plug with bare threads or for the screw-on terminal.

VDO5FMH - 120 Degree Long Elbow Type, 10mm and 12mm Adaptable Plug Thread Size, 5 ohm Resistance, For Terminal Stud, Rubber Sheathed, Compact Type

Or

VD05EMH - 120 Degree Long Elbow Type, 10mm and 12mm Adaptable Plug Thread Size, 5 ohm Resistance, For Terminal Nut or Integrated Terminal, Rubber Sheathed, Compact Type

Can someone take a quick look at their engine and tell me if

• 120 Degree Elbow is correct
• What type of connection to top of sparkplug

Thanks in advance…
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Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 750

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father - CFI) - USAAF 1st Lt. Captain B-17H
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  #4  
Old 09-27-2021, 11:29 AM
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rvbuilder2002 rvbuilder2002 is offline
 
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The factory standard spark plug and plug boot on the 912 has always been the versions that uses the threaded stud.

There is a pull off test prescribed in the inspection checklist for each hundred hour inspection that verifies a minimum pull off force for each plug boot. This thread demonstrates why following that is important. Once one starts getting a little bit loose and can be influenced by vibration it will cause wear rather quickly.
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Last edited by rvbuilder2002 : 09-27-2021 at 12:34 PM.
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  #5  
Old 09-27-2021, 02:16 PM
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Steve Melton Steve Melton is offline
 
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I'm happier every day with aviation plugs.
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  #6  
Old 09-27-2021, 03:50 PM
skydiverlv skydiverlv is offline
 
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Quote:
Originally Posted by Steve Melton View Post
I'm happier every day with aviation plugs.
Why? I have near 650 hours on my Bosch FR5-DTC plugs without missing a beat.
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  #7  
Old 09-27-2021, 04:20 PM
gasman gasman is offline
 
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Quote:
Originally Posted by Steve Melton View Post
I'm happier every day with aviation plugs.
You mean those old metric ones.........
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  #8  
Old 09-27-2021, 06:16 PM
Harvey rv12 Harvey rv12 is offline
 
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J3
I had the same experience you described. Engine running rough, pulled cowl, sure enough left front bottom plug wire hanging loose off spark plug. Upon close examination, it looks like the brass/bronze connector had worn from connecting & disconnecting numerous times. It became oval, out of round and would not stay on the plug. After replacing with same design NGK plug cap, I’m considering trying the alternative cap style that use the (thimble/nipple?) found on most automotive plugs.
But I think I will wait until the brain trust Has deliberated.
I must say it’s never easy getting plug wires off the plugs, that’s probably the reason rotax recommends that specific plug cap.

TVO
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  #9  
Old 09-27-2021, 10:19 PM
RFSchaller RFSchaller is offline
 
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I had one fall off. I discovered it on the ignition check during run up. I think it is easy to not get full engagement when you put the cap on which is why mine came off. Since then (about 200 hours ago) I am extra careful replacing them and have had no issues. I’ve got about 1150 hrs on the original boots in about 10 years. I replace all plugs at every annual (about 100 hours of service). They’re cheap, so why not?
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