If the bolt is designed to work with a nut you must predrill the wood to the exact size of the bolt or the the next closest size up of the drill bits at your disposal. If is a lag bolt designed to hold itself in the wood it will have large coarse threads and you only have to drill a hole aproximately 1/2 to 3/4 the size of the bolt in order to avoid splitting the wood. Note that the bigger the lag bolt the closer you have to bo to 3/4 of an inch
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