Threads/helicoil backing out

[aj4761]

Threads backing out of #4 top plug. Any insight to a solution/fix would be appreciated.

O320 ~1900 hrs SMOH

 

[Halleffect]

Have you considered Time-Serts?

 

[BobTurner]

This, of course, is not suppose to happen. The helicoil inserts have small "tangs" on the OD that dig into the aluminum and supposedly keep the coil from rotating.
Any good cylinder shop will have the tool for inserting a new helicoil. And there's enough sticking out that they should be able to remove the existing one without much effort. I'd guess 1 hour labor, max (unless baffling is in the way). Of course, at 1900 SMOH, any time anything comes up, you want to ask yourself if it's time for an overhaul. (e.g., when the shop removes this helicoil they may find the threads in the aluminum are now too deep.) Any other issues? If the coil has backed out enough that the plug won't torque down tight on the copper gasket, you should ground the plane now.

 

[lr172]

Looks to me like they are not backing out, which is rare, but instead the metal around the head threads are corroding and those upper threads are disappearing. Once that happens, the helicoil lifts up, making it look like it is backed out. Due to this metal erosion, you need to remove the helicoils and insert new ones deeper into the threads to avoid that area whercthe threads have eroded away.