Dimensions, drill sizes and wrench sizes for #6-40 UNF.
Designation
#6-40 UNF
Major (outside) diameter
3.505 mm · 0.138"
Pitch
0.635 mm · 40 TPI
Pitch diameter
3.093 mm · 0.1218"
Minor diameter (external)
2.726 mm · 0.1073"
Minor diameter (internal)
2.818 mm · 0.1109"
Recommended tap drill
#33 · 2.87 mm · ≈77% thread
Clearance hole
Close #23 · Normal #18 · Loose #7 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Socket key (across flats)
7/64"
Close #23 · Normal #18 · Loose #7 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Tightening torque estimate
SAE J429 tabulates grades from 1/4 in upwards, so no proof load or tightening torque is published for numbered sizes.
Put #6-40 UNF on a job sheet
The bolt schedule collects the fasteners for one job onto a single sheet - tap drill, wrench size, grade and tightening torque on every line, taken straight from pages like this one. Free schedules hold 3 lines and are saved in this browser only.
For a #6-40 UNF thread, drill the hole with a #33 tap drill (2.87 mm) before tapping. That produces roughly 77% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 3.505 mm and a pitch of 0.635 mm (40 TPI).
To pass a #6-40 UNF bolt through a part rather than thread it, use a clearance hole: Close #23 · Normal #18 · Loose #7 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice)..
All values are generated from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables - not hand-entered. See the full reference.
Frequently asked questions
What drill size do I need to tap a #6-40 UNF thread?
Drill #33 (2.87 mm) before tapping #6-40 UNF. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is #6-40 UNF a coarse or fine thread?
#6-40 UNF is a Unified fine (UNF) thread at 40 threads per inch. Fine pitches have a larger stress area than coarse at the same diameter and suit thin walls, fine adjustment and vibration-sensitive joints.
What size clearance hole does a #6-40 UNF bolt need?
Close #23 · Normal #18 · Loose #7 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the #6-40 UNF bolt pass through the part instead of threading into it.
What is the pitch diameter of #6-40 UNF?
The pitch diameter of #6-40 UNF is 3.093 mm (0.1218"). It is the diameter where the thread ridge and groove are equally wide, and the reference dimension for checking thread fit.
Nearest standard metric sizes by major diameter. These are substitutes by size only - the thread forms are not interchangeable, so never run one into the other.