20" × 2.5 TPI BUTT (pull type) External Thread Specifications
20-2.5 BUTT-2A · 20.0000-2.5 BUTT-2A
ASMESTD: ASME B1.9SOURCE: Table 3, Tables 1, 2, 5TYPE: External (bolt/screw)CLASS: 2AFLANKS 7° / 45°ROOT: roundTable 1 combination
Thread profile
20-2.5 BUTT-2A: solid = maximum material, dotted = minimum material, dashed = basic profile before the allowance. Flanks 7° (pressure) and 45° (clearance) from the radial line, 52° thread angle; drawn to scale.
Limits of size, class 2A
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter dCrest of the male thread, max = D − G | 20.0000 | 19.9891 | 19.9727 | 0.0164 |
| Pitch diameter EMax = D − h − G | 19.7600 | 19.7491 | 19.7327 | 0.0164 |
| Minor diameter KsRoot of the male thread; min is a reference value | 19.5200 | 19.4589 | 19.4095ref. | 0.0494ref. |
| Allowance GSpace below basic size; equals the Class 3 PD tolerance (Sec. 7) | 0.0109 | |||
| Root radius rRound root, male (Fig. 1a) | 0.028564 | 0.014280 | ||
| Thread engagement heh − G/2 at max, less the PD tolerance at min | 0.234550 | 0.218150 |
Dimensions in inches. Values are those of the standard (ASME B1.9); calculated cells follow the standard’s formulas. Where the standard prints a value that differs from its formula, the printed value is shown with a note.
Mating female thread, limits of size, class 2B
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter DnRoot of the female thread, min = D + 0.12542p; max for reference | 20.0000 | 20.0996ref. | 20.0502 | |
| Pitch diameter EMin = D − h | 19.7600 | 19.7764 | 19.7600 | 0.0164 |
| Minor diameter KnCrest of the female thread, min = D − 2h | 19.5200 | 19.5364 | 19.5200 | 0.0164 |
| Root radius rRound root, female (Fig. 1a) | 0.028564 | 0.014280 |
Dimensions in inches. Values are those of the standard (ASME B1.9); calculated cells follow the standard’s formulas. Where the standard prints a value that differs from its formula, the printed value is shown with a note.
Lead, flank angle and length of engagement
- Lead + flank variation limit Σ
- 0.01640 inPitch diameter equivalent of lead and flank errors together (Class 2: the PD tolerance; Class 3: half)
- Lead variation alone ±δl
- 0.00921 inAllowable lead error if there is no flank error
- 45° flank angle variation alone ±δα
- 2.158 °Half-angle error of the clearance flank
- 7° flank angle variation alone ±δα
- 4.556 °Half-angle error of the pressure flank
Gaging Sections 10, 11 · Tables 6, 7, 8, 11
Male thread 20-2.5 BUTT-2A
- Gage pitch tolerance (W)
- 0.00040 in
- Gage 7° flank angle tolerance
- 5 min
- Gage 45° flank angle tolerance
- 4 min
- Gage major / minor tolerance
- 0.0020 in
- Gage pitch diameter tolerance
- 0.00100 in
- PD equivalent of pitch and flank tolerance
- 0.0015 in
- GO pitch diameter limit
- 19.7491 in
- NOT GO pitch diameter limit
- 19.7327 in
- NOT GO truncated crest diameter
- 19.5927 in
- Setting plug truncated major diameter
- 19.9091 in
- Best-size wire
- 0.21659 · C = 0.32746 in, C
- Maximum wire
- 0.24573 · C = 0.41949 in, C
Female thread 20-2.5 BUTT-2B
- Gage pitch tolerance (X)
- 0.00060 in
- Gage flank angle tolerance
- 5 min
- Gage major / minor tolerance
- 0.0020 in
- Gage pitch diameter tolerance
- 0.00200 in
- PD equivalent of pitch and flank tolerance
- 0.0019 in
- GO pitch diameter limit
- 19.7600 in
- NOT GO pitch diameter limit
- 19.7764 in
- NOT GO truncated crest diameter
- 19.9164 in
- Plain gage tolerance
- 0.00100 in
- Root relief width GO
- 0.066800 in
- Root relief width NOT GO
- 0.100000 in
Notes
- Class 2: no taper/roundness requirement (para. 6.6.1).
Mates
20-2.5 BUTT-2A screws into a thread of the same standard at the same size and pitch. The standard mating internal thread is class 2B. Check the fit with the standard pair →
Working out the mating options…
Different pitch or a different thread family never mate; the fit check runs over both tolerance zones.
Related thread sizes
Same diameter other pitch, and neighbouring sizes
All values for 20-2.5 BUTT-2Aevery column of the workbook row
| Designation | 20-2.5 BUTT-2A |
|---|---|
| Decimal designation | 20.0000-2.5 BUTT-2A |
| Threads per inch | 2.5 |
| Thread form | BUTT |
| Series (Table 1) | BUTT |
| Series name | Pull type, rounded root (default) |
| External / internal | External |
| Class | 2A |
| Nominal size (in.) | 20 |
| Pitch P (in.) | 0.40000000 |
| Basic major dia D (in.) | 20.0000 |
| Size category | Table 1 combination |
| Source table | Table 3, Tables 1, 2, 5 |
| Printed page | 4, 5, 6, 10 |
| Major dia max (in.) | 19.9891 |
| Major dia min (in.) | 19.9727 |
| Major dia tolerance (in.) | 0.0164 |
| Major dia status | Toleranced: D - G (max), minus the PD tolerance (min) (paras. 6.2, 7) |
| Allowance G (in.; internal: mating external value, reference) | 0.0109 |
| Tolerance range (Tables 3-5) | Over 16 thru 24 |
| Range over (in.) | 16.0 |
| Range thru (in.) | 24.0 |
| Range mean dia for formula (in.) | 20.000 |
| Length of engagement Le = 10P (in.) | 4.0000 |
| Basic pitch dia E = D - h (in.) | 19.7600 |
| Pitch dia max (in.) | 19.7491 |
| Pitch dia min (in.) | 19.7327 |
| PD tolerance (in.) | 0.0164 |
| Basic minor dia K = D - 2h (in.) | 19.5200 |
| Minor dia max (in.) | 19.4589 |
| Minor dia min (in.) | 19.4095 |
| Minor dia tolerance (in.) | 0.0494 |
| Minor dia status | Max = D - 1.32542P - G; min is reference only = min PD - 0.80803P (para. 6.3, Sec. 4) |
| Root radius max (in.) | 0.028564 |
| Root radius min (in.) | 0.014280 |
| Root truncation s max (in.) | 0.033040 |
| Root truncation s min (in.) | 0.016520 |
| Engagement height he max (in.) | 0.234550 |
| Engagement height he min (in.) | 0.218150 |
| Lead + flank PD-equivalent limit (in.) | 0.01640 |
| Lead variation alone ± (in.) | 0.00921 |
| 45° flank variation alone ± (deg) | 2.158 |
| 7° flank variation alone ± (deg) | 4.556 |
| Lead angle at basic PD (deg) | 0.3692 |
| Lead angle (deg min) | 0° 22′ |
| Tensile stress area As (sq in.) | not given in the standard |
| Thread angle (deg) | 52 |
| Pressure (load) flank angle (deg) | 7 |
| Clearance flank angle (deg) | 45 |
| Flank orientation | Pull type (external thread pulls): 45° clearance flank leads, 7° load flank follows (Sec. 9) |
| Sharp V height H (in.) | 0.356256 |
| Basic thread engagement height h (in.) | 0.240000 |
| Thread height hs = hn (in.) | 0.265084 |
| Crest truncation f (in.) | 0.058128 |
| Crest width F (in.) | 0.065264 |
| Sweep cut: start diameter (in.) | 19.989100 |
| Sweep cut: tip diameter (in.) | 19.458900 |
| Sweep cut: depth (in.) | 0.265084 |
| Sweep cut: profile width at start diameter (in.) | 0.334736 |
| Sweep cut: tip width (root flat S max or radius r max) (in.) | 0.028564 |
| Constant c = 0.80803P (in.) | 0.323212 |
| PD tolerance component 0.00854√P (in.) | 0.005401 |
| W gage: pitch tolerance (in.) | 0.00040 |
| W gage: 7° flank tolerance (min) | 5 |
| W gage: 45° flank tolerance (min) | 4 |
| W gage: major/minor tolerance (in.) | 0.0020 |
| W gage: PD tolerance (in.) | 0.00100 |
| W gage: PD equivalent of pitch/flank tol. (in.) | 0.0015 |
| Z plain gage tolerance (in.) | 0.00100 |
| GO gage PD limit (in.) | 19.7491 |
| NOT GO gage PD limit (in.) | 19.7327 |
| NOT GO crest/minor truncated dia (in.) | 19.5927 |
| Setting plug truncated major dia (in.) | 19.9091 |
| Root relief width GO (0.167P) (in.) | 0.066800 |
| Root relief width NOT GO (0.25P) (in.) | 0.100000 |
| Best-size wire (in.) | 0.21659 |
| Measurement constant C, best wire (in.) | 0.32746 |
| Maximum wire (in.) | 0.24573 |
| Measurement constant C, max wire (in.) | 0.41949 |
| Check against formulas | OK |
| Notes | Class 2: no taper/roundness requirement (para. 6.6.1). |
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ASME B1.9: Buttress Inch Screw Threads. Source: workbook data (2026-10-10). Confirm against your licensed copy of the standard before use.