To avoid cross threading screw them in by Hand before tightening.
A Neverseeze type compound on the Threads can help the come out easier the next time.

Only Bosch or Beru Glow Plugs hold up well.

I had an experience where the Tips of 2 Autolite Glow Plugs became swollen and I barely got them out of the Cylinder Head.
I replaced them with Monark Glow Plugs and some of those failed in 6 months.

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84 300D, 82 Volvo 244Gl Diesel