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Assume 100 ft lb on the socket for impact driver and 300 ft lb on the socket for impact wrench, what's appropriate for driving 3" lag screws? Would an impact wrench break the screw?

If the torque is adjustable, the wrench seems like it's potentially more useful for re-use on cars. Also seems to be 50% heavier than impact drivers.

Update: it's a 5/16" screw and I broke it with a 100 ft lb impact driver so I guess for the purpose of the question, an impact wrench would definitely excessive.

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    Buy the little one and use it for everything. If it barfs on your lags, then stop doing it and pick up the hand tool. If you want to be all cool and nascar with 10"-1/2" lags, get the impact wrench. (FWIW, I don't think I've ever broken a lag with my impact wrench, but I predrill.) – Aloysius Defenestrate Dec 15 '15 at 2:44
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I think the answer to this will be "it depends" since there are different size lag bolts, there's the number of bolts you intend to install and how often, the size of the pilot hole you intend to drill, and you may have more or less usage for other tasks (cars) than other people.

The impact wrench will have more capacity, but may be less portable, heavier, and could over torque in some situations. The impact driver is likely to be more convenient, more likely to have interchangeable bits and batteries with your power drill and or other tools, but may not be up to driving hundreds of bolts per day. And if you're only doing a few bolts, the answer is neither and use a simple socket set to install them by hand. Regardless of which tool you use, a properly sized pilot hole will make the job easier, straighter, and less likely to split the wood.

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Neither. A plain, bog standard, ratchet.

Ratchet

Drill correctly sized pilot holes, don't over-tighten them, and you won't break the screws.

  • I'm a newbie so it might be a stupid question. Might be poor technique but I kept breaking bolts with manual socket tools even when I use the other had to counter balance it. It seems like since not all the force applied is torque (there's inevitably linear motion applied onto the bolt) and because there's a whole bunch of them to fasten, a power tool seems to make sense. – xster Dec 15 '15 at 3:08
  • I'll support @xster. I've torqued off several heads with a ratchet. The impact driver gives the torsional forces time to restore, which takes the steel out of the plastic region, making it much less likely to yield. – Chris Cudmore Dec 15 '15 at 14:24
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You'll want the wrench. You haven't said what the diameter of the screws is, and that's as important as the length, but from experience impact drivers are not useful for lag screws other than very small ones. For fasteners that large they're loud, slow, and will burn out if you work them that hard.

I do recommend piloting for lags, the size of the screw shaft for hard wood or slightly smaller for soft wood. Not doing so risks splitting the lumber, and you actually get a better hold. More thread makes contact with the wood.

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    FWIW, I built an entire deck using the lag screws (for the ledger, beams, and posts) with a cordless impact driver and it's no worse for the wear. – DA01 Dec 14 '15 at 21:46
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    Me, too. On about the fourth deck smoke started coming out of the DeWalt 14v driver I was using at the time. Shortly after it failed completely. The manufacturers themselves will tell that they're not designed for such heavy use. – isherwood Dec 14 '15 at 21:48
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I use my smaller driver on 1/4" and 3/8" lag screws in fir 2x4's & 2x6's the wrench may quickly strip your screws. The smaller driver will take a bit longer and mine is much lighter than the wrench,,, small driver gets my vote.

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First, always drill a pilot hole. secondly, after torquing the head off many over the past 30 years, I have found the best way is to buy a lag screw the same diameter but several inches longer than the screws you are going to use. Cut the head off and chuck it in a drill. Run it into the hole cutting the thread profile, then back out. Now you can easily run your real lag into the hole. Another old timer trick is to have a small container of liquid soap and dip the screw in that. It goes in much easier, and when the soap dries out it adds friction to the pull out value. Works for all screws, not just lags.

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    Interesting soap trick for screws in wood. I will have to give that a try. – ArchonOSX Dec 15 '15 at 8:43
  • Note that you don't need to drill pilot holes for a lot of lag screws. They're designed to be self drilling (not all, but many of them available today are). Lag bolts need to be predrilled, of course. – DA01 Dec 15 '15 at 17:17
  • @DA01 I was under the impression that lag bolt is a misnomer and that there's no such thing and there are only lag screws. – xster Dec 15 '15 at 19:28
  • @xster you're probably right. These are fuzzy terms. I think a "Lag Bolt" is officially a "Lag Screw" and what I'm talking about are actually "Structural Screws that are designed to be used where code requires Lag Screws". It's a good point. Let me update my answer. – DA01 Dec 15 '15 at 20:34
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For screws, an impact driver will work just fine.

For bolts, as Ecnerwal points out, you need to drill pilots, so there's really no need for an impact wrench for that, either.

UPDATE:

xster makes a good point that we should probably agree on terms first. The "traditional" lag bolt is, technically, not not a bolt at all, but a screw. Usually 1/2 dia for attaching ledgers and require washers be used with them:

enter image description here

You typically need to pre-drill for these to avoid splitting and just making them able to be screwed in. With proper pre-drilling, an impact driver should work fine, but there are arguments for going with an impact wrench.

Today, however, there are a lot of products that are designed to take the place of a traditional lag screw for things like attaching ledger boards:

enter image description here

These have an integrated washer and a narrower shaft and are designed to be drilled straight in without a pilot hole. For these, I've found an impact driver works just fine, and would actually worry that an impact wrench may actually have too much torque for the narrower shaft. In the case of this particular example product, Ledger Lock, the manufacturer points out that a cordless impact driver is all you need.

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    For the sake of science, I'll try and find the time soon to drive a bunch of 10" timberlok screws with the impact wrench and see if I can break any of them. – Aloysius Defenestrate Dec 16 '15 at 4:54
  • @AloysiusDefenestrate do report back! – DA01 Dec 16 '15 at 6:23
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    So, in the interest of science, I grabbed 6 of 10" long Timberlok brand screws. These are skinny (3/16), with screw threads only in the last 2" or so, The trusty impact wrench (DW059) happily zipped them into a mix of cross grain and end grain in my outdoor garden beds (SPF, a couple of years old, and a PT fence post). No breakage during normal fastening, and if I tried to overdrive them, they just stripped out and spun. Sorry that it turns out that science is relatively boring today. – Aloysius Defenestrate Dec 17 '15 at 1:26

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