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Auto plug adapter failure

DavidHarris

Well Known Member
Folks,

I had an unusual failure of an auto plug adapter at my condition inspection this month that I'd like to share in case anyone else experiences the same.

I built and fly a Vans RV7A with a Superior XP-IO360 engine and dual PMAGS. I use NGK BR8ES auto plugs with the EMAG-recommended short-reach 18 to 14 mm adapter that came with the engine. The aircraft has 250 hours and I’m in the midst of my 2nd condition inspection.

I removed the plugs and the adapters that came out with the plugs. I soaked the adapters in acetone to help clean a bit of residue. As I was hand-threading one of the adapters back into the cylinder with gentle hand force, the head and first 2.5 threads of one of the adapters sheared off. Thankfully the remaining threaded portion easily came back out of the cylinder.

I always use C5A antiseize on the bushings and plugs, and torque the bushings to 30-35 ft-pounds and plugs to 18 ft-pounds. My compression tests were in the mid-70s and the engine appears to be operating normally and has stayed in its temperature specs. I don't have a good explanation of why the metal adapter could have sheared with gentle hand force.

David Harris
 
Don’t do this

“torque the bushings to 30-35 ft-pounds and plugs to 18 ft-pounds.”

That is the problem. Read the PMAG manual and follow their recommendation to only torque the plug after hand tighten the plug into the bushing.
 
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