Left hand thread taps

[MattF]

Just on a tangent, if we're talking about glowplugs here, it isn't the same as drilling a hole in a bolt. There's going to be a central hole down the outer threaded stem already, filled with the conductor for the tip. Personally, I'd get it done professionally. This is one of those jobs where you're liable to cause yourself far more expense and time than you'd save getting someone else to take the pain, (for a price, obviously).