Because as previously stated the Glow Plugs sea on the tapered shoulder right where the Element comes out of the Plug if you nicked the threads a little while drilling there may be enough threads to use it the way it is since the Torque is not much.

The disadvantage of any drilling is that if it gets off center the Glow Plug May not seal on the Shoulder correctly and Carbon may build up between the Glow Plug Body and the Glow Plug Hole.
Because of the above reason which ever thread restoration Kit has the best Pilot Drill might be a good choice.

A Helicoil in Aluminum has no strength issues and I believe it removes the least parent Metal and if you have the correct set breaking off the bottom Tab should not be an issue.

If for some reason you believe your Glow Plug might not seal at the shoulder you can slide a thin Copper Crush Washer over the Tip of the Glow Plug that has an ID and OD to match. If you heat that Washer red hot and cool it will become soft.

Fuel Injection Shops should have some washers that would work as they get more then they need in the Fuel Injection Pump Gasket Kits and ACE Hardware may have them.

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84 300D, 82 Volvo 244Gl Diesel