Thread - Pilot Drill Charts
 

Hello All,

Just wondering if there is some form of "Quick Reference Guide" or chart available; online or elsewhere, which identifies what size thread or drill should be used when working with different material. For example, when drilling a pilot hole for a 6 G screw into wood what size should the pilot drill be?

Once upon a time this information used to be printed on the screw's packaging. This information is getting rarer to find each time I go into a hardware store.

Also, is there some form of table that shows if a piece of steel rod of a certain dimension needs to be threaded - what size thread is the most suitable for the application?

Similarly, a chart that shows what size tap to use to cut a thread into a specific drill diameter?

Any assistance would be greatly appreciated.

Kind regards
Lionel

Can't be that hard to find, surely? Chart

Lionelgee, next time you go to your engineers supplies place for a bearing or a can of CRC ask for a "tap drill chart" Sutton or who ever does drills, will probably produce one.

Hi Lionel,

With reference to wood screws in softwoods, it is not that critical to just eyeball a drill size the same or slightly larger than the minor diameter of the screw (diameter at the bottom of the threads). With hardwoods, use a larger drill with a diameter up to half the depth of the threads. I have broken off woodscrews in hardwood by using too small a pilot hole. With a set of fractional and metric drills you will always find a good size. If you find the screw very tight going in just back it out and run the next larger size drill down the pilot hole.

The diameter of holes for tapping and diameter of rods for threading are always determined for the specific tap or die and not the other way round. There is some tolerance with threading a rod slightly oversize, but little or no tolerance for a hole undersize for tapping. A broken tap will likely ensue.

Studying a tapping and drilling chart will show you what is possible.

Hope this is of some help.

Cheers,

Jacques, one that I have wanted to do in the last few days was 5/16 UNF.
It requires a 6.8mm or 6.9mm hole. Not found in the standard metric drill set (0.5mm steps) and nothing close in imperial.
So there are a few sizes that really could do with their own special purpose drill bits.
:teach:On the tight screw thing, (please stay on topic) I was told (years ago) to run the threaded end through your hair. (does not apply to everyone) :devil:
I have found that it does make a difference.

Letter and number drill sizes
 

Hi Lynn,

Just checked my Machinery's Handbook and see what you mean. The attached shows a 5/16 UNF tap requiring a Letter I drill: 0.272"

Not the kind of thing you would get at the local hardware store.

Number and letter size drills derived from American and British wire gauge sizes and are still sometimes used in American industry.

17/64" might be too small and risk tap breakage. 9/32" would not leave a lot of thread.

A 7.0 mm drill might work. Converting 7.0 mm to inches is 0.2756" which is only 0.0036" larger, about the thickness of printer paper, and might suffice.

I inherited the attached partial set of number drills. Sets like that or letter drill sets may turn up at swap meets.

Rubbing a wood screw on a block of soap also aids screwing it in. Oops! off again.

Cheers,

Jacques, I used the 17/64 drill and tried a tap which didn't feel good, so tried another which felt somewhat better. I tapped 8 holes. It worked but will plan to buy a 6.8mm bit at some point in the future.
In N.Z. Patience and Nicholson make drill bits. I think they are an off shoot of the Australian drill maker (lost the name for the moment) anyhow the put out some very good charts: one a tap-drill chart and the other a drill selection chart. together very useful.
There are some horrible and useless drill bits on the market these days.

Hello All,

Thank you for all the replies and suggestions. The last time I saw a table of the different sizes for threads and pilot holes was in the back of my high school Year 10 Metal Work Textbook. That was quite some time ago.....

It was the year where Love is Like Oxygen - by The Sweet and Lay Down Sally by Eric Clapton was in the Australian Top 100 Songs

Kind regards
Lionel

Slight diversion- Number drill sizes and carburetor jets
 

Good Day All,

Just a slight diversion from topic but which many may find interesting.

After posting a picture of my numbered drill set I remembered that Ford carburetor jet sizes are given in number drill sizes. I know jet sizes have been covered before on MLU, but I thought I would see if any of my number drills matched an old Ford Holley carburetor jet I kept.
Many are missing from the set but #54 was there and a quick micrometer check confirmed it. The jet is numbered 054 which is the drill size.
I see on another MLU post the Ford CMP truck jet size was 050 except for the Workshop truck which was 053. Some American Flathead Ford sites specify 051 as standard, probably for cars. Quite a few sizes are available and it is a separate subject in its own right, but I thought others may find this interesting especially those new to the old Ford engines or just old engines in general.

So numbered drills are still being used in manufacturing, at least for replacement carby jets.

Cheers,

Not quite. The Holley Jet sizes closely correlate to thousandths of an inch; as the numbers go up, the Jets get bigger. Numbered drill sizes are related to wire gauges, as the numbers go up, the Drills get smaller. It was just a happy coincidence that your Number 54 drill fit a #54 Jet, as that is the point the two tables cross over.

BTW, never resize a jet with a drill bit. You will never get an accurate hole in the size you want. The WW2 Flathead Holley carburettors used jets and power valves that were standardised and are still used today in the latest Holley Quad Bore carbies. You can usually find a kit with a variety of different jet sizes to play with for a few dollars in most Auto Parts stores.

Carburetor jet sizes
 

Tony,

Thanks for that information. Especially the availability of new Holley parts.
I stand corrected on the jet sizing misinformation. My bad.

Cheers,