I'm looking for something to drill a 3" + 1" counterbored hole by first drilling with a 1" auger bit and then drilling with a 3" bit part of that way yet keeping it perfectly aligned. I found https://www.amazon.com/dp/B00FXCGEY6/ which seems like it can replace the self feed screw insert in https://www.amazon.com/gp/product/B00005JRJM, but I couldn't find anything else (I was hoping to find a longer pilot guide because precision matters here).

Is there another name for this type of tool or something else I should be looking up other than counterbore? I've tried various searches but no luck.

I suppose I could potentially make this work using a shorter auger bit as the guide or even making a combo bit to drill both at once, but am wondering what the best thing to use is here.

EDIT: it'll be going into a tree to install a TAB / GL bolt for a treehouse so I don't have the benefit of being able to use a press unfortunately

enter image description here

enter image description here

enter image description here

(First two images from Amazon links and third from https://en.wikipedia.org/wiki/Counterbore)

  • 5
    The simpler approach is to bore the 3" hole first using any normal bit that leaves a pilot hole the 1" can follow - trivial with Forstner bits, for instance.
    – Ecnerwal
    Jun 2, 2022 at 15:00
  • @Ecnerwal The challenge with that approach is I'll be installing a massive 1-1/4" bolt (1" ID) with 3" collar about 6 inches into oak, so any slight misalignment will make it next to impossible to get in there.
    – g491
    Jun 2, 2022 at 15:10
  • 1
    Sounds like you are planing to use a hand drill. If needing it to be exact, would suggest finding a friend/neighbour with a drill press/ milling machine so not depended on attachments for alignment.
    – crip659
    Jun 2, 2022 at 15:35
  • 1
    How about drill a pilot hole with, say, a 1/8” bit, then the 3” Forster bit as far as necessary and finally the 1”?
    – DoxyLover
    Jun 2, 2022 at 16:03
  • 2
    And, this is why it's important to specify the use case in the question up front... Between Ecnerwal and DoxyLover, you've got your answer, maybe one of them will write it up.
    – FreeMan
    Jun 2, 2022 at 16:13

1 Answer 1


The normal process for drilling a counter sink is to drill the hole first, then put the (tapered) countersink bit in. However, for a counter bore you want to do the opposite.

To make your counterbore, you'll want to drill the larger counterbore first using something like a Forstner or spade bit, then use the divot left by the bit's pilot spur to center your smaller bit's pilot spur to make the rest of the hole. To get them perfectly aligned, you'd usually do this on a drill press ("pillar drill" or "post drill" outside the US) where you can take your time to get everything nicely lined up then clamp down your work piece.

Since you're doing this on a tree, moving it to the drill press will be a bit difficult.

  • You could drill a pilot hole for both bores with a long but small diameter bit, maybe 1/8" diameter. Something big enough to not break at that length and deep enough to get the main bore started in a straight line, yet small enough to accurately capture the pilot spur on the bits you're using. Use this to center both the counterbore and, afterwards, the main bore.
  • If you're creative with some ratchet straps, you could, potentially, strap a drill guide (something of a "poor man's drill press") to the tree.
    • This should keep the drill centered on the same spot without the pilot hole and keep the drilling motion all in the same plane, allowing you to use the "drill press method" described above of drilling the counterbore first.
    • You would probably need to strap down the guide very securely and test it (give it a few tugs, make sure it isn't going anywhere) before you start drilling.
    • Despite your confirmation that it won't move, still be very gentle and accurate with the drill to not dislodge it when you're drilling.
    • With the vagaries of naturally occurring bark on a naturally growing tree, it might will require some shims to get the hole to line up in exactly the direction you need it to go into the tree. i.e. just setting it against the bark is no guarantee your hole will line up with whatever is being attached to it. This will take a lot of fiddling to make it right, but should give you an accurately lined up counterbore.

It sounds like the main bore is 1" diameter and the counterbore is 3" diameter. I'm not sure what you're putting into the counterbore, but whichever method you choose, you might consider moving the counterbore bit size up some to accommodate the possibility that they won't be precisely concentric. By making it bigger, whatever sits in the counterbore may be off center in the counterbore, but will have room to be precisely centered over the main bore. I'd imagine that something will cover this, hiding your fudge space so nobody but you will know. If not, well, your secret is out, but at least the tree house will be safely secured to the tree, so who cares.

  • The 3" piece is called a collar and the intent is that it perfectly fit the hole so that it distributes the massive force over more surface area of the tree.
    – g491
    Jun 3, 2022 at 16:13
  • 2
    Well, that negates the "bigger hole" possibility and just means that your setup has to be more accurate. Perhaps consider some test holes on some scrap material to get your technique down.
    – FreeMan
    Jun 3, 2022 at 16:20
  • 3
    Drill the big hole, insert the collar, drill the small hole using the collar as the guide.
    – Ecnerwal
    Oct 31, 2022 at 14:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.