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I tapped a hole in my motor (to hold the valve cover) but I don't think the pre-existing hole was deep enough. Can you make a tapped hole deeper? As the drill bit for the pilot hole should fit right between the threads, this should work, correct? Or am I flirting with destroying the threads I created?

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You can do this, but you could possibly run into two problems (as I see it):

First, what you said about destroying the threads you already created.

Second, you run the risk of running into an oil or water channel when doing this kind of drilling. I don't think there is a great risk of it in the head, but there is a risk to be aware of.

As an aside, can you shorten your original bolt and still have it hold enough to keep the valve cover on? This might be a better option, if it's viable.

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    I ended up doing this successfully. I was careful to keep the drill perfectly straight. I started drilling slow and found the bit caught on the existing threads, but if I 'gunned' the drill very fast it seemed not to catch and dug deeper into the head. As for your second point, I knew I was good on how deep I could go as I checked the dimensions of the engine. I just didn't know how deep I needed to tap until I had already done so and realized it was not enough... – ngramsky Dec 23 '14 at 2:56

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