I may add some tip in removing the glow plugs w/o stripping is to bring the engine to operating temp, this may be difficult especially the glow plugs are under the intake manifold and also lightly tap the glow plug (using a socket) to help loosen it up some carbon before removing... optional using an electric impact tool that can adjust the torque amount (lowest setting) the hammering action of it can help loosen it too,

i had to replace the GP on the Mercedes bluetec engine, and they came out like it was nothing when the engine was at operating temp.