I use NGK split fire plugs too.Let engine be cold before working on plugs.Change one plug and one wire at a time.Remove wire blow with compressed air in plug hole.So no flies fall in hole.Gap. each plug and replace with new one.I go hand tight with aluminum heads.
Proceed to next hole.
do blow holes out.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt