Dimensions, drill sizes and wrench sizes for 2-1/4-8 UN.
Designation
2-1/4-8 UN
Major (outside) diameter
57.15 mm · 2.25"
Pitch
3.175 mm · 8 TPI
Pitch diameter
55.088 mm · 2.1688"
Minor diameter (external)
53.255 mm · 2.0966"
Minor diameter (internal)
53.713 mm · 2.1147"
Recommended tap drill
1" · 25.4 mm · ≈770% thread
Clearance hole
Close 1" · Normal 1" · Loose 1" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Close 1" · Normal 1" · Loose 1" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Tightening torque estimate
SAE J429 defines no grades above 1-1/2 in - its tables stop there - so no proof load or tightening torque is published for this size. Fasteners this large are normally specified to ASTM A193, A320 or an equivalent rather than to a J429 grade.
Put 2-1/4-8 UN 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 2-1/4-8 UN thread, drill the hole with a 1" tap drill (25.4 mm) before tapping. That produces roughly 770% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 57.15 mm and a pitch of 3.175 mm (8 TPI).
To pass a 2-1/4-8 UN bolt through a part rather than thread it, use a clearance hole: Close 1" · Normal 1" · Loose 1" - 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 2-1/4-8 UN thread?
Drill 1" (25.4 mm) before tapping 2-1/4-8 UN. That leaves roughly 770% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 2-1/4-8 UN a coarse or fine thread?
2-1/4-8 UN is in the UN constant-pitch series: it runs 8 threads per inch, a pitch shared across a range of diameters rather than scaled with the size.
What size clearance hole does a 2-1/4-8 UN bolt need?
Close 1" · Normal 1" · Loose 1" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 2-1/4-8 UN bolt pass through the part instead of threading into it.
What is the pitch diameter of 2-1/4-8 UN?
The pitch diameter of 2-1/4-8 UN is 55.088 mm (2.1688"). 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.