Date registered: Nov 2014
Vehicle: 1990 350SDL, "Grandpa's Roadster" Project Car
Location: New Hampshire
Mentioned: 9 Post(s)
Quoted: 800 Post(s)
The steel thread material is harder than the aluminum head material. No way you can run a tap in there with the threads still in place and get good results. The tap will seek the softer aluminum and you'll end up with a loose coil of steel in the hole. Ideally you can use some penetrating oil and a sharp pick to unwind the steel from the hole, then you may be good to go, or you are in good shape to use the heli-coil. Otherwise, figure out what size drill the helicoil tap calls for and drill it out that big. With any luck all the steel will come out at the same time. If that doesn't work, you can use a different insert, Time-sert, for example, that uses a larger hole. Just make sure there's enough material so you don't strike water.
"Real engines don't have spark plugs!"