UNFfine60° flank
#12-28 UNF Thread Dimensions
#12-28 UNF is a fine pitch UNF thread. There are 28 threads per inch, giving a pitch of 0.91 mm. The recommended tap drill leaves roughly 75% thread engagement.
Nominal Ø5.49 mm
Pitch0.91 mm
Threads per inch28
Pitch Ø-9.48 mm
Tap drill-17.55 mm
Thread profile
millimetres unless stated- Designation
- #12-28 UNF
- Thread system
- UNF
- Series
- fine
- Flank angle
- 60°
- Major diameter
- 5.49 mm / 0.22"
- Pitch
- 0.91 mm
- Threads per inch
- 28
- Pitch diameter
- -9.48 mm
- Minor diameter (external)
- -22.78 mm
- Minor diameter (internal)
- -19.46 mm
- Thread depth
- 14.13 mm
- Recommended tap drill
- Ø -17.55 mm / -0.6911"
How these values are derived
Basic ISO thread geometry uses the fundamental triangle height H = 0.866025 × P. External minor diameter d₃ = d − 1.226869 × P, internal minor diameter D₁ = d − 1.082532 × P. Tap drill is given as nominal diameter minus pitch, which approximates a 75% thread engagement — adjust for tough materials or deep holes.