I ALWAYS use a glow plug reamer when I replace a set of plugs. You also need to crank the engine after you ream each glow plug hole to blow the loosened carbon out. There are two types of reamers, one type has the universal attached to it for #s 1-4 and there is another type used for the #5 hole which looks like a glow plug itself and threads into the hole.

Additionally, ALWAYS use some copper anti-seize paste on the threads of the glow plugs. Wurth CU1100 is very good.

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Regards,
Aaron