Dimensions, drill sizes and wrench sizes for 2-1/2-8 UN.
Designation
2-1/2-8 UN
Major (outside) diameter
63.5 mm · 2.5"
Pitch
3.175 mm · 8 TPI
Pitch diameter
61.438 mm · 2.4188"
Minor diameter (external)
59.605 mm · 2.3466"
Minor diameter (internal)
60.063 mm · 2.3647"
Recommended tap drill
2-3/8" · 60.33 mm · ≈77% thread
Clearance hole
Close 2-33/64" · Normal 2-17/32" · Loose 2-9/16" - 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 2-33/64" · Normal 2-17/32" · Loose 2-9/16" - 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.
2-1/2-8 UN in numbers
2-1/2-8 UN is 2.5000 in across the crests with 8 threads per inch, a 3.175 mm pitch. The basic pitch diameter is 61.438 mm and the bolt's minor diameter 59.605 mm, so the thread is 1.948 mm deep on the bolt. Its tensile stress area is 4.4421 in², 11% more than the coarser 2-1/2-4 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 2.4082 and 2.4164 in, and a class 2B nut between 2.4188 and 2.4294 in, so even the largest bolt clears the smallest nut by 0.0024 in. The nearest metric size, M64 × 6.0, is 0.50 mm larger across the crests and its pitch is 2.825 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 63.5 mm and P = 3.175 mm. A smaller hole leaves more thread; the recommended 2-3/8" drill above gives about 77%.
Tap-drill hole diameters for 2-1/2-8 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
61.03
2.4026
65%
60.82
2.3945
70%
60.61
2.3863
75%
60.41
2.3782
Class 2A and 2B limits
The commercial fit for 2-1/2-8 UN: class 2A for the bolt, class 2B for the nut or tapped hole. Inches.
Class 2A external and class 2B internal limits of size for 2-1/2-8 UN.
Dimension
Min
Max
2A major diameter
2.4826
2.4976
2A pitch diameter
2.4082
2.4164
2B pitch diameter
2.4188
2.4294
Matches the ASME B1.1 class 2A/2B table for this size, as printed in the data sheets of American Fastener (Cap Screws & Bolts - External Threads Class 2A) and American Fastener (Nuts - Internal Threads - Class 2B). Basic pitch diameter E = D − 0.649519 P = 2.4188; the allowance between basic and the 2A limits is 0.0024. The B1.1 formula, class 2A pitch tolerance T = 0.0015 D^(1/3) + 0.0015 √D + 0.015 P^(2/3) with the allowance 0.3 T and the 2B tolerance 1.3 T, gives the same figures for this size. Minor-diameter limits are not given here.
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 2-1/2-8 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 2-1/2-8 UN thread, drill the hole with a 2-3/8" tap drill (60.33 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 63.5 mm and a pitch of 3.175 mm (8 TPI).
To pass a 2-1/2-8 UN bolt through a part rather than thread it, use a clearance hole: Close 2-33/64" · Normal 2-17/32" · Loose 2-9/16" - 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 2-1/2-8 UN thread?
Drill 2-3/8" (60.33 mm) before tapping 2-1/2-8 UN. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 2-1/2-8 UN a coarse or fine thread?
2-1/2-8 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 2-1/2-8 UN bolt need?
Close 2-33/64" · Normal 2-17/32" · Loose 2-9/16" - 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 2-1/2-8 UN bolt pass through the part instead of threading into it.
What is the pitch diameter of 2-1/2-8 UN?
The pitch diameter of 2-1/2-8 UN is 61.438 mm (2.4188"). 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.