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I'm trying to drill pilot holes for 5/16" x 3.5" lag bolts. Based on charts like this one I need a 9/64" hole that is 3.5" deep. Looking around the Internet (always a mistake), the best I can find is a 9/64" x 6" "aircraft extension" bit. Home Depot can get them in two weeks. My local hardware store doesn't have it all. So:

  • Am I looking for the right bit (9/64 x 3.5)?
  • What is an "aircraft extension"?
  • Should this bit be hard to find?
  • I need to drill 6 holes into redwood. Should I expect to break a bit or two?

Thank you!

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  • Any decent hardware store stocks longer bits. Search "jobber length", those are longer twist drills... – Jimmy Fix-it Nov 23 '19 at 6:52
  • one of the reasons why I prefer stuctural screws to lag bolts is that there's no need to drill. – Jasen Nov 24 '19 at 3:59
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One of those "aircraft bits" should work fine for you. Just because the bit is 6" long doesn't mean you have to actually "use" all six inches. You should be able to find quality bits from multiple sources on the internet. (Maybe not Amazon...) You might find some locally at professional-level tool vendors if not at the BigBox stores. (A little help)

I wouldn't expect to break any bits, but I suppose it could happen. Everybody has bad days. Since you may not be able to jump in the car and pick up another one, you might order an extra. (Maybe if you don't use it you could send it back.)

Aircraft extension bits get their name from their use primarily in the aircraft industry in the years coming up to WWII.

Aircraft drill bits are long drill bits originally designed for use in the aircraft industry where they often need to drill holes reaching through deep cavities, and then through thin materials. Here's more about them.

The pilot hole size you'll want will be something like 9/64" or 3∕16" depending on the hardness and moisture content of the wood you're drilling into. Both of these factors can affect the fit of the screws in the pilot holes. Here's a write-up that goes into more detail about it.

I'd do a test or two starting out with the smaller diameter bit and make the hole larger if it proves to be too snug to drive the screw easily. It's always easier to make the holes bigger than it is to make them smaller.

Another idea: There are self-drilling lag screws manufactured to not need pilot holes. They tend to be more expensive, but from what I understand they are roughly structurally equivalent. I've used these, Spax 5/16 x 6" on a couple of projects with satisfying results. This may be an alternative for you, maybe not.

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  • I considered structural screws but since I'm using this to hang a TV mount I think I want to stick with the lag screws they gave me (and, presumably, tested). – LoftyGoals Nov 23 '19 at 6:16
  • There's not much difference in structural.screws and lag bolts or lag screws... Mostly just the names. What's significant is the diameter and threads. The presence of a shank can influence the choice as well. Secondarily the head shape and size. The self-drilling screws should be plenty sturdy for a TV if you're fastening to wall studs. If you're fastening to flat wood panels, structural screws without a shank might be the better choice because of more thread area, etc. – Greg Nickoloff Nov 23 '19 at 12:38

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