| If the nut is a 10mm, then the stud will probably be a 6mm, 12mm nut usually has an 8mm stud. Heli-coil is pretty straight forward, just have to be careful when you use the tap. Aluminum tears instead of cutting because it is so soft. I usually use either silicone spray or something called "Alumi-tap" to coat the threads of the tap. If the hole is all the way through the casting, I would pull the plenum. Any bits of shavings will get into the engine if it drops into the air chamber. Get the proper kit at the local advance or autozone (about $30) and buy the drill bit recommended on the kit package. Next hold the drill plumb and level as you drill down. Put some grease around the drill bit, it will grab the shavings or have someone hold a vacuum on it to suck them up. Control of the shavings is paramount.
Next, put a piece of tape around the bit to act as a depth guage. Place it on the bit where you want to stop. Keep the drill steady while you drill out the hole.
Then, start to tap the hole. I usually tap a little at a time, take it 1/4 of a turn at a time, back it the same, then go that 1/4 plus another quarter. With aluminum it is important to keep the shavings clear of the thread. So I remove the tap often and clean it, recoat with silicone and go again. Work slowly and carefully until you have the hole tapped down to a depth of about 1/2 in or so (measure the length of the helicoil) . Clean the hole out completely of shavings, then test with a bolt to ensure the threads are completely tapped and smooth. Next, take the heli-coil insert tool and helicoil. thread it into the hole with a constant clockwise turning motion. Do not turn counterclockwise. I also coat the threads with some oil to smooth the insertion. Stop when the helicoil is flush with the surface. The tool will back out with a counter-clockwise rotation. That's it, your done.
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