[fast507]

If the threads were installed dry, it's most likely a bi-metallic corrosion of the steel reacting with the aluminum. Another possibility is galling due to heat and vibration. Very common in aircraft applications. Most steel fasteners that are intended to be threaded into aluminum are coated with Cadmium (gold colored) which acts as a sacrificial coating to the threads. Cadmium degrades into a very soft oxide where as aluminum degrades into a very hard oxide. Once the thin plating wears away, and it doesn't take long, you have a very good possibility of galling and corrosion. Helicoils are a common fix for this but run the risk of galling or corroding in the same fashion making a very bad leak. Stepping up the threads to a 14mm and using a slightly larger plug does the same job, but with fewer pieces. Good thing you found this with the motor out of the frame!!!

A little dab of graphite grease, Moly lube, or Nev-R-Seez will stop this in the future.