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Thread: Pilot hole for 3/8 in. lag screw

  1. #1
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    When drilling into a hardwood, such as oak. What size pilot hole should be drilled for a 3/8 inch lag screw? The length of the lag screw will be about 6 inches.

  2. #2
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    Originally posted by Dale Genther:
    When drilling into a hardwood, such as oak. What size pilot hole should be drilled for a 3/8 inch lag screw? The length of the lag screw will be about 6 inches.
    The drill bit should be the same diameter as the root of the screw. The root is the shaft of the screw exclusive of the threads. Most just eyeball it by holding the drill bit up to the screw until you find one that matches the root diameter. You'll want to grease the screw with soap, anhydrous lanolin or something similar before driving it of course.

  3. #3
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    With a little research I've answered my own question. It shoould be 1/4 in for softwoods and 17/64 for hardwoods

  4. #4
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    You may wish to drill full diameter (or slightly less) for the length of the shank of your lag bolts. The shank of course is the un-threaded portion of the lag. In other words, step drill for the two significant diameters that you are dealing with. By not drilling for the shank diameter (full or a hair less) you are running the risk of splitting your wood when you drive the shank in, especially if you are close to the end of the piece you are drilling. Try a few test runs on some available scrap if you can. Good luck.

  5. #5
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    All of the above, plus beeswax is the best lubricant for screws and lags, and a lubricant is obligatory if going into hardwood. Don't neglect to lubricate the shank as well as the threads.

  6. #6
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    I always use a 5/16 for pilot in both Fir and Oak if the lag is HDG. Bronze lags are a little sharper and will tolerate a little tighter hole but 1/4" is much to small. A long lag often will sheer with such a tight pilot. As said, you must use a 3/8 top pilot to match the length of the shank. Use full diameter lags and not those with undersize shank and rolled threads.

  7. #7
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    Regarding screw lubricant: I had always used a block of beeswax. A year or two ago, someone here said he used the soft wax toilet-sealing rings for screw lube. I tried it and it can't be beat.

    Don't use soap, because the chemicals in the soap can cause the screws to corrode. And spit is only a little less corrosive.

    Wayne

  8. #8
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    Wayne: Right on. I think I was the first on this forum to suggest using toilet rings for lubricating fasteners. Then some wag suggested that we use used ones at our own discretion.

  9. #9
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    How about using Slick Seam? It is mostely Bee's wax, and it is pretty easy to get, etc etc. Would there be any reasons not to use Slickseam?

    Noah

  10. #10
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    I kept some beeswax aboard for lubricating screws. Then one day I went to drive some screws and it had disappeared somewhere in the clutter. In desperation I eyed up the Ivory liquid until I remembered the Slick Seam. I worked fine. So I'll fess up to using it ever since.

  11. #11
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    Thanks, Bayboat. I couldn't recall whom to credit for that handy toilet-ring tip, but I knew I heard it here! I've now got a couple of them in my shop. Who sez we old dogs can't learn new tricks?

    Wayne

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