Dimensions, drill sizes and wrench sizes for 2-20 UN.
Designation
2-20 UN
Major (outside) diameter
50.8 mm · 2"
Pitch
1.27 mm · 20 TPI
Pitch diameter
49.975 mm · 1.9675"
Minor diameter (external)
49.242 mm · 1.9387"
Minor diameter (internal)
49.425 mm · 1.9459"
Recommended tap drill
1-61/64" · 49.61 mm · ≈72% thread
Clearance hole
Close 2-1/64" · Normal 2-1/32" · Loose 2-1/16" - 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 2-1/64" · Normal 2-1/32" · Loose 2-1/16" - 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.
2-20 UN in numbers
2-20 UN is 2.0000 in across the crests with 20 threads per inch, a 1.27 mm pitch. The basic pitch diameter is 49.975 mm and the bolt's minor diameter 49.242 mm, so the thread is 0.779 mm deep on the bolt. Its tensile stress area is 2.9904 in², 20% more than the coarser 2-4.5 UNC at the same diameter. The nearest metric size, M52 × 5.0, is 1.20 mm larger across the crests and its pitch is 3.730 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 50.8 mm and P = 1.27 mm. A smaller hole leaves more thread; the recommended 1-61/64" drill above gives about 72%.
Tap-drill hole diameters for 2-20 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
49.81
1.9610
65%
49.73
1.9578
70%
49.65
1.9545
75%
49.56
1.9513
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 2-20 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 2-20 UN thread, drill the hole with a 1-61/64" tap drill (49.61 mm) before tapping. That produces roughly 72% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 50.8 mm and a pitch of 1.27 mm (20 TPI).
To pass a 2-20 UN bolt through a part rather than thread it, use a clearance hole: Close 2-1/64" · Normal 2-1/32" · Loose 2-1/16" - 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 2-20 UN thread?
Drill 1-61/64" (49.61 mm) before tapping 2-20 UN. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 2-20 UN a coarse or fine thread?
2-20 UN is in the UN constant-pitch series: it runs 20 threads per inch, a pitch shared across a range of diameters rather than scaled with the size.
What size clearance hole does a 2-20 UN bolt need?
Close 2-1/64" · Normal 2-1/32" · Loose 2-1/16" - 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 2-20 UN bolt pass through the part instead of threading into it.
What is the pitch diameter of 2-20 UN?
The pitch diameter of 2-20 UN is 49.975 mm (1.9675"). 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.