hi,
I am doing for a friends engine head who has done same thing. The procedure is very simple and easy.
Take a brass dowel of the whole size, let say whole size is 6mm, dowel size 6.2 mm . drill it for .1mm and the insert the dowel in glow plug whole. Now that the dowel is in , drill it for the required size and tap it.

I hope that helps........


Sharad