I am going to use this screw to join 2x4s together. How can I select the correct bit size for this screw?

enter image description here

Product image from homedepot.com (linked above)

4 Answers 4


That's basically a #2 PSD2-2 drive. It's designed for relatively high torque driving that would make a normal Phillips drive cam out while minimizing the chance of stripping that you'd have with a square drive. Wild (and cynical) speculation, but I'm guessing the manufacturer calls it "Unidrive" because Phillips holds a patent on it. I'd get a #2 PSD2-2 driver (also called Phillips-Square drive, etc.):

enter image description here

Image courtesy of fastenal.com

They're common enough that most of the big-box home improvement stores carry them, but oddly uncommon enough that a lot of the time the screw manufacturer will sometimes include one in the box.


A #2 Philips or #2 Robertson (square drive) will do the job. Source: Spax faq #2:

Q) What does Unidrive mean? A) Unidrive is a( Phillips/Square combo) drive system for the SPAX fastener. The user has the option of either using a Phillips drive bit or a square drive bit to install the SPAX fastener.



If you have a micrometer, you can measure the inside diameter of the screw; that's the size bit (or a bit smaller) that you want to use to drill your pilot hole. If not, you can hold it up against a tape measure and guesstimate that diameter. It doesn't have to be perfect, but you definitely do not want to drill a pilot hole the same diameter has the outside diameter of the screw.

  • wtf??? who pilots a screw in a 2x4???? May 4, 2016 at 0:01
  • Depends on the size of the screw and type of 2x4. I've split more than one using a size 12 screw in redwood.
    – BillDOe
    May 4, 2016 at 0:17
  • fair enough i guess May 4, 2016 at 13:58
  • Actually, my answer was meant to suggest how to correctly determine the size of a pilot hole, not whether or not one is needed, so your comment puzzles me. It seems the comment would have been more appropriately addressed to the OP.
    – BillDOe
    May 4, 2016 at 22:52
  • 1
    now i see the confusion - the way i read the op's post was he was looking for how to choose the right driver bit. you read it as he was looking for how to use the right drill bit. thats why you talked about the root diameter measurement and ensuing drill hole. i (and it seems everyone else who addressed it) read it as what the driver bit size was. my original comment was from-the-hip shock at predrilling a structural screw in a 2x4 (which is how i interpreted the use of the screw and a 2x4 - a structural connection) which is what a diy guy may do, but a professional contractor wouldn't May 5, 2016 at 15:57

I've been using exactly the same type of screws as shown in the OP. #2 Robertson worked, but #2 Phillips did not. In the latter case, both the Phillips bit (I use Wera Impaktor) and the screw head were stripped.

I have since switched to the dedicated bit made by SPAX and carried by HD. To find it, go to HD.com and search for "unidrive bit". You can also find similar bits at amazon by searching "Sq/Phillips Combination Power Screwdriver Bit".

Your Answer

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

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