BIZOMAT MAKEOS · Fasteners
Thread Length of Engagement
How long must the thread engagement be so the bolt breaks before the threads strip?
Minimum length of engagement for a metric or unified thread so that the external thread does not strip before the screw breaks, with the extra length needed when the tapped material is weaker.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Basic major diameter | mm | Nominal size of the thread. |
| Pitch | mm | Thread pitch (for unified threads, 25.4 ÷ threads per inch). |
| Maximum minor diameter of internal thread (0 = basic) | mm | From the thread limits table for your thread class (e.g. 6H); 0 uses the basic minor diameter, which is slightly optimistic. |
| Minimum pitch diameter of external thread (0 = basic) | mm | From the thread limits table for the bolt class (e.g. 6g); 0 uses the basic pitch diameter. |
| Minimum major diameter of external thread (0 = basic) | mm | Minimum major diameter of the bolt thread from the limits table; 0 uses the basic size. |
| Maximum pitch diameter of internal thread (0 = basic) | mm | Maximum pitch diameter of the tapped thread from the limits table; 0 uses the basic pitch diameter. |
| Tensile strength of the screw | MPa | Minimum tensile strength of the screw (e.g. 800 MPa for property class 8.8). |
| Tensile strength of the tapped material | MPa | Minimum tensile strength of the nut or tapped component material. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Basic major diameter
- What it is
- The basic major diameter of the thread, the nominal size in the designation. With the pitch it gives the basic pitch and minor diameters and the stress area.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the thread size, not the tap drill or the clearance hole size.
What this means: Pitch
- What it is
- The axial distance from one thread crest to the next. For a unified thread it is 25.4 divided by the threads per inch.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 20 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Do not enter threads per inch here; convert TPI to a pitch in millimetres first.
What this means: Maximum minor diameter of internal thread (0 =…
- What it is
- The largest allowed minor diameter of the tapped (internal) thread for its tolerance class, from the thread limits table. It sets the shear area of the bolt thread.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 uses the basic minor diameter, which gives a slightly optimistic, shorter engagement length.
What this means: Minimum pitch diameter of external thread (0 =…
- What it is
- The smallest allowed pitch diameter of the bolt (external) thread for its tolerance class, from the thread limits table. It sets the bolt thread shear area.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 uses the basic pitch diameter. For very high-strength screws the tool also uses this value to work out the stress area.
What this means: Minimum major diameter of external thread (0 =…
- What it is
- The smallest allowed major diameter of the bolt (external) thread for its tolerance class. It sets the shear area of the tapped (internal) thread.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 uses the basic major diameter; real bolts are slightly smaller, so a class limit gives a safer result.
What this means: Maximum pitch diameter of internal thread (0 =…
- What it is
- The largest allowed pitch diameter of the tapped (internal) thread for its tolerance class. With the bolt major diameter it sets the internal thread shear area.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 uses the basic pitch diameter. Enter all four class limits for a design check; the tool flags results that used basic sizes.
What this means: Tensile strength of the screw
- What it is
- The minimum tensile strength of the screw or bolt material, from its property class or material specification.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 800 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A stronger screw raises the strength ratio J, so a longer engagement is needed when the tapped material governs.
- Get it right
- Use the minimum ultimate tensile strength, not the proof or yield strength.
What this means: Tensile strength of the tapped material
- What it is
- The minimum tensile strength of the nut or of the part the thread is tapped into, from the material specification or supplier data.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 310 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A weaker tapped material raises J and lengthens the required engagement.
- Get it right
- For a tapped casting or aluminium part use that material’s strength, not the strength of a steel nut.
Method and formulas
- At = π/4 (d − 0.938194P)² (or π(Es min/2 − 0.16238P)² above 1240 MPa).
- External thread: Le = 2At ÷ [π Kn max (½ + 0.57735 n (Es min − Kn max))], with n = 1/P.
- Internal thread shear area per unit length: π n Ds min [½ + 0.57735 n (Ds min − En max)]; J = (As external × Su screw) ÷ (As internal × Su tapped); required length = J × Le when J > 1.
- Example: M10 × 1.5 (8.8) into 310 MPa material needs about 10.888 mm (1.0888 × d) of engagement.
Limitations
- Basic dimensions (inputs left at 0) overstate the shear areas slightly; use the limits for your thread class for a design check.
- Assumes full-form threads over the whole engaged length; countersinks, chamfers and incomplete threads at the hole bottom do not count.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set tapped hole depth in aluminium, cast iron or soft steel
- Check a short nut or thin plate thread
- Decide whether a thread insert is needed
Who it is for
- Design engineers
- Maintenance fitters
- Quality inspectors
What you get
- Tensile stress area
- Engagement to protect the external thread
- Strength ratio J and engagement for the internal thread
- Required engagement in diameters
Interpreting the result
- If the required length is impractical, use a thread insert, through bolt and nut, or a stronger tapped material.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1536–1537. Topic reference; no source tables or recommended settings are reproduced.
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1528. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Threads, tapped holes and screw lengths
In Bolted Joints by the Numbers
What it covers: How long a thread must engage so the screw breaks before the thread strips, how to design a tapped hole, and how to choose a screw length that puts full thread where it is needed.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.