Dimensions
Wrench sizes follow ISO 4014 / ISO 4017. Older DIN 931 / DIN 933 bolts differ at several sizes (M10 = 17, M12 = 19, M14 = 22, M22 = 32) - check the bolt before choosing a spanner.
ISO 273 clearance holes (fine / medium / coarse).
M20 × 2.5 in numbers
M20 × 2.5 is 20 mm across the crests with a 2.5 mm pitch, which is 10.2 threads per inch. The basic pitch diameter is 18.376 mm and the bolt's minor diameter 16.933 mm, so the thread is 1.534 mm deep on the bolt. The nearest inch size, 3/4-10 UNC, is 0.95 mm smaller across the crests and its pitch is 0.040 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 20 mm and P = 2.5 mm. A smaller hole leaves more thread; the recommended 17.5 mm drill above gives about 77%.
Tightening torque estimate
Target preload is 75% of proof load; torque is estimated from T = K · F · d.
Strengths are ISO 898-1:2013 Table 3 for d > 16 mm. Class 8.8 is size-banded - 580 MPa proof at or below 16 mm, 600 MPa above - while 10.9 and 12.9 hold one value across the range.
These figures assume a nut factor of K = 0.20 (dry / as-received steel). K captures thread and head friction and can swing torque by 30% or more - a lubricated joint needs noticeably less. Change the lubrication assumption in the bolt-torque calculator. Put M20 × 2.5 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.
What tap drill do I need for M20 × 2.5?
For a M20 × 2.5 thread, drill the hole with a 17.5 mm tap drill (17.5 mm - the standard D − P rule) before tapping. That produces roughly 77% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 20 mm and a pitch of 2.5 mm (≈10.16 TPI (converted)).
To pass a M20 × 2.5 bolt through a part rather than thread it, use a clearance hole: Close 21 · Normal 22 · Loose 24 mm.
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 M20 × 2.5 thread?
Drill 17.5 mm before tapping M20 × 2.5. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is M20 × 2.5 a coarse or fine thread?
M20 × 2.5 is a metric coarse thread. Coarse is the default pitch for M20: when a drawing or supplier says just M20, it means this 2.5 mm pitch.
What size clearance hole does a M20 × 2.5 bolt need?
Drill 21 mm for a close fit, 22 mm for a normal fit or 24 mm for a loose fit (ISO 273). A clearance hole lets the M20 × 2.5 bolt pass through the part instead of threading into it.
How much torque should I use on a M20 × 2.5 bolt?
Estimated tightening torque for M20 × 2.5 runs from 441 N·m (Class 8.8) to 713 N·m (Class 12.9), targeting 75% of proof load with a dry nut factor of K = 0.20. Lubricated threads need noticeably less torque.