Need to drill holes in double top plate

How long should my bits to be? 9 inch or 18 inch
is auger bits the best choice (0.5-0.75 diameter)?
Thx!
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A garden variety spade bit works fine

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However you may find that a bellhanger's bit works best, they are about 16 inches long, look like a twist drill and have a hole in the tip of the bit to attach cords or wires so they can be pulled through the hole as you withdraw the bit. Should be available most anywhere. When sharp they cut fast and can handle the occasional nail without any problem.

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A standard double top plate is 3-3.25" thick in modern construction and no more than 4-4.5" if built with rough sawn lumber.
I like the auger bits but a cheap spade bit will make a few holes before you burn it up and will cost about 1/3 the price. The auger bits take a bigger drill motor so if you are using a 3/8 use the spade bits.
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