To determine the proper drill size for 75% thread on a special pitch tap use the following formula:
[Major Diameter of tap, in inches] - (.975 divided by the number of Threads Per Inch) = Drill Size [in 1000ths of an inch.]
To find the drill size for a 1/4-32 tap:
.250" [major diam of tap, in inches: 1/4" = 1 divided by 4 = .250"] - (.975 / 32 tpi) = X drill size for 75% thread
.250" - (.0305) = .2195"
Once you calculate that number, go to our 'Drill Sizes and Decimal Equivalents'
reference page and find the closest drill size match- you probably won't find a
perfect match, so you'll have to choose to drill either a bit big or a bit small.
.2195" on the drill chart is close to 7/32" (.2188", a bit low) or #2 drill size(.2210", a bit high.)
Metric tap drill sizes are the diameter minus the pitch (For example, 42mm x 1.5 tap uses a 40.5 drill).
Follow these links to browse our taps and die selection:
For common National Coarse and National Fine sizes we offer hand taps, spiral point taps, spiral flute taps,and oversize taps, as well as round adjustable dies, and hex rethreading dies.
For making metric right hand threads we supply metric taps and metric dies.
You can buy individual sizes or tap and die sets.
To make left hand threads, choose inch size left hand taps and dies or metric left hand taps and dies.
To make pipe threads use pipe taps and dies.
We carry an extensive stock of special thread taps and special thread dies.
To cut square threads we offer acme taps.
In addition to the many sizes we list, custom taps to your specifications can often be shipped in as little as 24 hours.
Follow this link to order metric drill bits or inch size jobber length drill bits.