This is the proper way to tap a hole. First, determine what size the hole is already. Measure with a dial caliper or micrometer. Or just use your tape measure, and take a guess.
When tapping a fresh hole, you need to drill it to the proper size. Now, each tap size has a specific drill bit size associated with it. Like a 5/16-18 tap needs a "F" drill bit. I don't remember the numerical equivalent to that letter drill bit.
So, if you can't get an accurate measurment for whatever reason, get a drill bit set out, and find a bit that will barely go in the hole, nice and snug. Now you know that the drill bit for the tap has to be bigger than that drill bit.
So, once you find a tap and drill bit that will work, lay the intake upside down, so the hole is perfectly straight up. Make some wedges, duct tape it in place, whatever will keep it perfectly still.
So, once you have the right tap, drill bit, and the intake is rock solid and the hole is pointing perfectly straight up, drill the hole down about 1" or so. If you can drill all the way through, that is preferable, as then you can tap it all the way through. If you chose the bit size right, it will take only a little bit of metal away on all sides. Make sure you are perfectly aligned with the hole. Having someone watch you drill is helpful also to keep it straight.
So you have your hole drilled, now take your tap, get a cup full of motor oil, a bunch of rags, and a metal pick tool that is perfectly straight. Get your tap, put the tap in the tap handle (they make T-handles specifically for taps, get one that will fit this size, it helps a lot)
Now dip the tap in the oil and stick it the hole. The tap is tapered, so it will go in a little bit, then stop. Make sure the tap is perfectly aligned, and make a 1/2 turn, putting firm, but not hard pressure on the tap. You should feel some resistance while turning, and this is normal, as you are cutting out the aluminum.
Now back the tap out until it's loose, and remove it. Clean all of the metal filings and oil off with a rag. Take your metal pick tool, dip it in oil, and stick it in the hole and move it around. The metal will stick to the oil, and that will clean the hole out. The prevents problems, like stuck taps or crappy threads.
Then dip the tap into the oil, and put it back in the hole. Now, turn it the 1/2 turn you already did, and then do another 1/2 turn. Then back out the tap, clean it off, clean the hole, and repeat this until you tap the entire hole.
If at any point it feels like there is more than normal resistance, back the tap up immediately, clean the threads, and try it again. If you keep getting to the same point where there is too much resistance, you need to try cleaning out the threads more, or making sure your tap is going in straight. If it still has too much resistance, and you have enough threads, just stop there. No reason to risk breaking the tap.
If the tap binds, and you turn to hard, the tap will snap, and you will have a 5 dollar tap stuck in a $400 intake.
But as long as you take your time, and only go an additional 1/2 turn each time, you will do fine. The tap will spin freely when you are in already tapped threads, so you will feel when you get to fresh untapped metal. I learned this the hard way, so please heed my words, as I only speak from experience. And getting a broken tap out is not easy.
Then when you are done, find a bolt to fit it, slowly thread it in to test it. The bolt should go in like butter. Aluminum is extremely soft, and very easy to strip.
So you shouldn't have to put a wrench too it until it's flush with the intake. You don't need to tighten it much either. Maybe 10 ft-lbs at the most. So once you know the bolt goes in smoothly, put some thread sealer on the bolt, and stick her in.....
But you have the right idea to plug that hole. Best thing to do in that situation.
Scott
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