Dimensions, drill sizes and wrench sizes for 2-1/4-16 UN.
Designation
2-1/4-16 UN
Major (outside) diameter
57.15 mm · 2.25"
Pitch
1.588 mm · 16 TPI
Pitch diameter
56.119 mm · 2.2094"
Minor diameter (external)
55.202 mm · 2.1733"
Minor diameter (internal)
55.432 mm · 2.1823"
Recommended tap drill
2-3/16" · 55.56 mm · ≈77% thread
Clearance hole
Close 2-17/64" · Normal 2-9/32" · Loose 2-5/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table.
Close 2-17/64" · Normal 2-9/32" · Loose 2-5/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table.
2-1/4-16 UN in numbers
2-1/4-16 UN is 2.2500 in across the crests with 16 threads per inch, a 1.588 mm pitch. The basic pitch diameter is 56.119 mm and the bolt's minor diameter 55.202 mm, so the thread is 0.974 mm deep on the bolt. Its tensile stress area is 3.7638 in², 16% more than the coarser 2-1/4-4.5 UNC at the same diameter. The nearest metric size, M56 × 5.5, is 1.15 mm smaller across the crests and its pitch is 3.912 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 57.15 mm and P = 1.588 mm. A smaller hole leaves more thread; the recommended 2-3/16" drill above gives about 77%.
Tap-drill hole diameters for 2-1/4-16 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
55.91
2.2013
65%
55.81
2.1972
70%
55.71
2.1931
75%
55.60
2.1891
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 2-1/4-16 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 2-1/4-16 UN thread, drill the hole with a 2-3/16" tap drill (55.56 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 57.15 mm and a pitch of 1.588 mm (16 TPI).
To pass a 2-1/4-16 UN bolt through a part rather than thread it, use a clearance hole: Close 2-17/64" · Normal 2-9/32" · Loose 2-5/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table..
All values come from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables. See the full reference.
Frequently asked questions
What drill size do I need to tap a 2-1/4-16 UN thread?
Drill 2-3/16" (55.56 mm) before tapping 2-1/4-16 UN. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 2-1/4-16 UN a coarse or fine thread?
2-1/4-16 UN is in the UN constant-pitch series: it runs 16 threads per inch, a pitch shared across a range of diameters rather than scaled with the size.
What size clearance hole does a 2-1/4-16 UN bolt need?
Close 2-17/64" · Normal 2-9/32" · Loose 2-5/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table. A clearance hole lets the 2-1/4-16 UN bolt pass through the part instead of threading into it.
What is the pitch diameter of 2-1/4-16 UN?
The pitch diameter of 2-1/4-16 UN is 56.119 mm (2.2094"). 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.