---
type: "thread"
name: "M42×4.5"
designation: "M42×4.5"
series: "metric-coarse"
series_name: "ISO Metric Coarse"
system: "metric"
size_label: "M42"
pitch_in: 0.177165
pitch_mm: 4.5
basic_major_dia_in: 1.65354
basic_major_dia_mm: 42
basic_pitch_dia_in: 1.53846
basic_pitch_dia_mm: 39.077
basic_minor_dia_in: 1.46177
basic_minor_dia_mm: 37.129
tap_drill: "25 mm"
tap_drill_dia_in: 0.98425
tap_drill_dia_mm: 25
tap_drill_thread_pct: 290.8
clearance_normal_drill: "25 mm"
tensile_stress_area_sqin: 1.737414
tensile_stress_area_sqmm: 1120.91
internal_class_assumed: "6H"
standard: "ISO 68-1, ISO 261, ISO 262, ISO 965-1, ISO 898-1"
verified: "August 1, 2026"
url: https://shopmath.org/threads/metric-coarse/m42x4.5
---

# M42×4.5 Thread — Dimensions, Tap Drill & Clearance

The tap drill for M42×4.5 is 25 mm — 0.9842 in (25.000 mm) — which leaves 290.8% thread engagement with a cut tap. Basic pitch diameter is 1.5385 in (39.077 mm) and basic minor diameter is 1.4618 in (37.129 mm). Tapped-hole figures on this page assume a class 6H thread; drill sizes for 50–85% engagement are in the tap drill table below.

- **Series**: ISO Metric Coarse
- **Designation**: M42×4.5
- **Pitch**: 4.500 mm
- **Tap drill**: 25 mm
- **Standard**: ISO 68-1, ISO 261, ISO 262, ISO 965-1, ISO 898-1

## Basic Thread Dimensions for M42×4.5

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 1.6535 | 42.000 |
| Pitch | 0.1772 | 4.500 |
| Basic pitch diameter | 1.5385 | 39.077 |
| Basic minor diameter | 1.4618 | 37.129 |
| External thread root diameter | 1.4362 | 36.479 |
| Sharp V thread height (H) | 0.1534 | 3.897 |

_Basic dimensions are the theoretical values before any tolerance class is applied._

## Tap Drill Sizes for M42×4.5

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 25 mm drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 1.5385 | 39.077 | 25 mm | 0.9843 | 25.000 | 290.8 |
| 60% | 1.5155 | 38.493 | 25 mm | 0.9843 | 25.000 | 290.8 |
| 65% | 1.5039 | 38.200 | 25 mm | 0.9843 | 25.000 | 290.8 |
| 70% | 1.4924 | 37.908 | 25 mm | 0.9843 | 25.000 | 290.8 |
| 75% (default) | 1.4809 | 37.616 | 25 mm | 0.9843 | 25.000 | 290.8 |
| 80% | 1.4694 | 37.323 | 25 mm | 0.9843 | 25.000 | 290.8 |
| 85% | 1.4579 | 37.031 | 25 mm | 0.9843 | 25.000 | 290.8 |

_Target hole Ø is the theoretical hole for the stated engagement. The drill column is the nearest size in the standard number/letter/fractional/metric index, and actual thread % is what that real drill produces._

## Clearance Hole Sizes for M42×4.5

The through-hole a M42×4.5 fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | 25 mm | 1.6929 | 43.000 | Located joints, dowelled assemblies |
| Normal | 25 mm | 1.7716 | 45.000 | General assembly — the default |
| Loose | 25 mm | 1.8898 | 48.000 | Slotted or adjustable joints, weldments |

_Clearance holes per ISO 273 (metric) and ASME B18.2.8 (inch)._

## Thread Class Limits for M42×4.5

Manufacturing limits for classes 6H. A gauge reading outside these bands is a reject, not a judgement call.

### Internal thread (nut / tapped hole) limits

| Limit | Class 6H (in) | Class 6H (mm) |
| --- | --- | --- |
| Major Ø min | 1.6535 | 42.000 |
| Pitch Ø max | 1.5509 | 39.392 |
| Pitch Ø min | 1.5385 | 39.077 |
| Pitch Ø tolerance | — | 0.315 |
| Minor Ø max | 1.4882 | 37.799 |
| Minor Ø min | 1.4618 | 37.129 |

## Tensile Stress Area of M42×4.5

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 1.73741 | 1120.91 |

The effective cross-section used to size a M42×4.5 fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

## Other Thread Pitches in the Same Size as M42×4.5

A M42 fastener is not always M42×4.5. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [M42×3](https://shopmath.org/threads/metric-fine/m42x3.md) | ISO Metric Fine | 3.000 mm | 25 mm |

## Sources and Method for M42×4.5

### Basic dimensions

- **Source**: ISO 68-1 basic profile and ISO 898-1 tensile stress area; verified in docs/research/data-sourcing.md §3.2; diameter/pitch pairs: ISO 261 / ISO 262 diameter-pitch pairs via Wikipedia (CC BY-SA 4.0); cross-checked against threadlib metric_thread.csv (BSD-3) and FreeCAD CAM metric-internal-6H.csv (LGPL-2.1+); clearance holes: ISO 273 (metric) and ASME B18.2.8 (inch) via FreeCAD FeatureHole.cpp (LGPL-2.1+), cross-checked against bd_warehouse clearance_hole_sizes.csv (Apache-2.0)
- **Method**: computed+compiled
- **Verified against**: threadlib metric_thread.csv (BSD-3-Clause) and FreeCAD CAM metric-internal-6H.csv (LGPL-2.1+), agreeing cell-for-cell — 215 checks passed at 0.001 mm

### Tap drill sizes

- **Source**: Machinery's Handbook %-thread formula (geometry-derived); nearest-standard-drill-to-75% rule verified at 36/39 published chart rows in docs/research/data-sourcing.md §3.4
- **Method**: computed
- **Verified against**: D-P rule verified as exactly the 77.0% thread case

### Class limits

- **Source**: threadlib metric_thread.csv (BSD-3-Clause) and FreeCAD CAM metric-internal-6H.csv (LGPL-2.1+), agreeing cell-for-cell
- **Method**: compiled
- **Verified against**: two independent published datasets agreeing cell-for-cell to 0.001 mm; ISO 965-1 formulas not located, so these are compiled not computed

### Formulas

**Tap drill hole diameter, metric threads**

```text
hole Ø  =  D − (%thread ÷ 100) × 1.29904 × P

  D = basic major diameter (mm)
  P = pitch (mm)
```

_1.29904 = 2 × 0.64952, twice the engaged thread height per side on the 60° ISO profile._

**Actual thread engagement of the chosen drill**

```text
%thread  =  (D − drill Ø) ÷ (1.29904 × P) × 100
```

_The drill column rounds to a real drill; this is the engagement that drill actually leaves._

**Tensile stress area, ISO metric threads**

```text
As  =  (π ÷ 4) × ((d₂ + d₃) ÷ 2)²

  d₂ = basic pitch diameter
  d₃ = external thread root diameter
```

_ISO 898-1 form, evaluated on unrounded intermediates._

### Assumptions

- The headline tap drill assumes 75% thread engagement, the general shop default. Every engagement from 50% to 85% is in the tap drill table — change the assumption and the number moves.
- Tap drill figures are for cut taps. Drill diameters are nominal; a worn or run-out drill cuts oversize and reduces engagement further.
- Tapped-hole figures assume a class 6H internal thread, the general-purpose fit.
- Dataset scope: EXCLUDED: ISO 965-1 tolerance-grade formulas (not located — data-sourcing.md open item 2), 6g external limits (single-sourced), form tap drills, M68-M100 (verified demand cliff at M64).

**Data last verified**: August 1, 2026

## Common Questions About M42×4.5

### What is the tap drill size for M42×4.5?

The tap drill for M42×4.5 is 25 mm — 0.9842 in (25.000 mm). That leaves 290.8% thread engagement with a cut tap.

### What is the pitch diameter of M42×4.5?

The basic pitch diameter of M42×4.5 is 1.5385 in (39.077 mm). For a class 6H thread it runs 1.5385 in (39.077 mm) to 1.5509 in (39.392 mm).

### What is the minor diameter of M42×4.5?

The basic minor diameter of M42×4.5 is 1.4618 in (37.129 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill.

### What is the thread pitch of M42×4.5?

M42×4.5 has a pitch of 4.500 mm, the distance from one thread crest to the next.

### What size clearance hole does a M42×4.5 fastener need?

close fit 25 mm (1.6929 in (43.000 mm)), normal fit 25 mm (1.7716 in (45.000 mm)), loose fit 25 mm (1.8898 in (48.000 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of M42×4.5?

The tensile stress area of M42×4.5 is 1.73741 in² (1120.91 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in ISO Metric Coarse: [M39×4](https://shopmath.org/threads/metric-coarse/m39x4.md) — tap drill 25 mm
- Larger in ISO Metric Coarse: [M45×4.5](https://shopmath.org/threads/metric-coarse/m45x4.5.md) — tap drill 25 mm
- All ISO Metric Coarse sizes: [ISO Metric Coarse thread chart](https://shopmath.org/threads/metric-coarse.md)
- Every thread series: [Screw thread reference](https://shopmath.org/threads.md)
- Cross-series chart: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)

---

*Source: [ShopMath](https://shopmath.org/threads/metric-coarse/m42x4.5) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*