# Pilot holes- Selecting the appropriate drill bit diameter



## Tedstor (Mar 12, 2011)

if you ever need a quick way to guage the diameter of a bolt/screw for a pilot hole, I found this trick while browsing some old volumes of popular mechanics. Probably old news to many. But might be useful to some. I could have used this trick a few weeks ago had I known about it. Yes- i could have used a caliper gauge, but my adjustable wrench is more readily available. Anyway….......


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## JoeinGa (Nov 26, 2012)

Yep, good tip. Sometimes an adjustable wrench is BETTER, because the arms on most calipers are so thin that it's difficult to get a good reading where the flutes are cut in a drill bit.


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## Charlie5791 (Feb 21, 2012)

A pilot hole is much smaller than the bolt/screw diameter. It's used as a guide for the larger hole that WILL be for a bolt/screw. Normally that would be called a clearance hole  And normally it's drilled slightly larger than the bolt diameter so the bolt clears it.

It's early and I think I'm just pickin' nits. Need more coffee…. hehehe


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## SnowyRiver (Nov 14, 2008)

To determine the pilot hole size, I just hold up the screw and then hold a bit over the top of the threaded portion of the screw, I pick a bit that just covers the shank of the screw, but not the threads. This will work everytime.


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## debianlinux (Jul 27, 2013)

I thought I was the only one who thought it was just fine to hold the bit up to the screw and finger guage it or eyeball it.


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## firefighterontheside (Apr 26, 2013)

I hold up the drill bit to the screw and choose. For soft woods like pine I will choose a bit that's quite a bit smaller than the screw, but for hardwoods I will choose one the same size as the shank of the screw.


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## Loren (May 30, 2008)

That's good, especially with metal. Wood is obviously a little
forgiving.

I hold the screw in front of the drill shank and eyeball it
usually, allowing for the tolerances the material I'm
drilling needs to get the effect I want (loose, tight, 
easy driving, hard driving, etc.).


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## redSLED (Mar 21, 2013)

Judging diameters - I close one eye and check with my other eyeball - never had a problem with this method yet. No need for tools, computers or gurus. Some of Murphy's Laws may apply.


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## mechanicalengineer (Sep 27, 2010)

I keep my drill bits in a drill box (container) and just use the holes that hold the the drill bits as a gauge for what size drill to use. Whatever hole the bolt will fit in, that's the size drill I use.


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## curliejones (Mar 12, 2012)

Yep! I do the eyeball to shank method and find it just fine. Another 2 cents worth = Lag bolts are usually for larger scale projects, but are a great fastener if used properly. Lots of us are doing construction along with the finer crafts. I find I usually recess the head into the lumber that I'm assembling, so I size the first drill bit to the washer I'm using under the head of the lag bolt. Usually using a spade bit, I drill the first hole by the hairy eyeball method estimating the thickness of the lag bolt hex head plus the washer. Next I'll drill a hole sized to match the unthreaded shank portion of the lag bolt, wrapping the twist drill bit with masking tape at the proper depth after measuring the unthreaded shank length of the lag. The third time down the hole is with a twist drill bit again marked with tape at a point to match the threaded portion of the lag up to and NOT INCLUDING the tapered point. This third depth is usually 1/4" to 1/2" shorter than the threaded portion of the lag bolt shank, depending on the size you are dealing with. The diameter of this third hole matches the shank only and not the threads, allowing the threads to cut their way into the wood. This is, of course, most efficiently accomplished with three different drills when multiple holes are needed to avoid the years wasted changing the drill bits back and forth. I draw the line at three, but a good powerful driver might be handy for actually installing the lag once the hole is ready. I usually use a ratchet and socket to get a little exercise. (If the fit seems tight, try passing one side of the lag across a bar of soap prior to installing it.) Before I starting reading LJs, I thought that it was drills (instead of clamps) that you could never have too many of!


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## curliejones (Mar 12, 2012)

Oh! and judging diameters - 1) hold the spade bit across the washer 2) eyeball the unthreaded shank next to the drill bit and 3) eyeball the shank portion, ignoring the threads next to the drill bit. For machine bolts or part 2 of this process, I keep handy a couple of the plastic gauges with various-sized holes in them, in case the hairy eyeball is having a fuzzy day.


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## dbhost (Jul 20, 2009)

FWIW, a sizing gauge, just a simple piece of plastic with a mess of different sized holes with their sizes on them works well when trying to determine bolt / screw sizes. For clearance holes, the hole should be just a hair bigger than the bolt. For pilot holes, it should be smaller, with enough material left in the hole for the screw to bite into and hold on well…

Most hardware stores / home improvement stores sell bolt sizing gauges for a dollar or so…


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## Doe (Aug 26, 2010)

A couple of dumb questions. If you use the wrench, shouldn't you measure the shank? I think the threads stick out a bit more, but maybe that would still be too big. What to the numbers mean for screws? There should be a translation table somewhere that says for a number 8, use a xxx drill bit. By the way, I eyeball it with an occasional annoying result.


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## curliejones (Mar 12, 2012)

Hey Doe, this comes from a popular fastener e-tailer. I find that in construction efforts in soft lumber (SYP)









the screw size correlates with the drill bit in 64ths. Such as a number 8 screw requires a hole from an 8/64th" (1/8") bit, #9 9/64th, #10 a 10/64th (5/32") bit. I also dimple the wood with a 1/2" twist drill to recess the head slightly. The following shows 2 tables for different screw types and recommends tapered drill bits. I don't own any of those but recognize that would be a good answer to the tapered form of standard wood screws for furniture assembly.


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## curliejones (Mar 12, 2012)

And Doe, I believe the threads only stick out as far as the unthreaded shank. The lower shank is smaller in size where the threads were cut out of a rod that was consistent in diameter. Therefore, the wrench should work and be easy enough to check by "wrenching" both the upper shaft and the threaded portion to assure they are the same diameter.


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## Tedstor (Mar 12, 2011)

Just now getting back to this (poorly titled) thread. I actually found this method useful for "clearance holes", not "pilot holes", as Charlie pointed out.
I also could have used it a few weeks back when I was trying to drill some holes for some dowel rods- of unknown size. I'm pretty sure the dowels were sold as 3/8', but as it turned out, they were actually closer to 5/16".


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## curliejones (Mar 12, 2012)

Greetings Tedstor! I've had the same problem with dowels. I guess I'll have to buy metric bits because some in the marketplace don't match SAE measure, though they are marked as such.


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## REO (Sep 20, 2012)

LOL i have been watching this one to see how it developed. I thought the OP was looking for clearance hole but waived off to see. I have seen something like this used for quick reference for threaded holes. use the same size drill for the bolt and use the adjustable to gage the smallest diameter of the drill.(touching four points across the cutting edge of the flutes and the relief edge of the flutes. then choose a drill that will fit in this dimention. I agree its very dirty but it does work. I typically just count down three spaces in the drill index. Four for wood or plastic.


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## Doe (Aug 26, 2010)

Thanks very much, I am enlightened in the mysteries of pilot holes. curliejones, I will be printing the chart for future reference and keep it near the screw storage bins; I appreciate the info and will be taking out for a test drive soon. Although I like the wrench business because I don't have to remember anything.


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