KEVOS® MAKEOS · Fasteners
Bolt and Screw Grip Length Check (Grip Gauging Length, Body Length, Full Thread in the Joint)
For a bolt or socket head cap screw of a given nominal length, what is its grip gauging length and body (shank) length by the standard rules, will the full thread start inside the parts it clamps so the nut or tapped hole can pull it tight, and does enough thread stand beyond the joint for the nut or the thread engagement I need?
Thread length rules for bolts and screws from the handbook: for metric bolts and screws the maximum grip gauging length Lg = L − B (basic thread length); for socket head cap screws the grip length LG = L − LT, the total thread length LTT = LT + 5P and the body length LB = L − LTT (the same form as Lg max = L − b, Ls min = Lg max − 5P for BS 4168 screws); for heavy hex structural bolts LG max = L − LT and LB min = LG max − Y max, with the bolt threaded full length when LB min is no more than 2.5 thread pitches (sizes to 1 in) or 3.5 pitches (larger). The grip gauging length is the minimum design grip: the clamped thickness should be at least this long so the nut or the tapped hole works on full thread, and the length beyond the joint is checked against the nut height or the engagement you need.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Fastener | choice | The kind of fastener, which sets the thread length rule and the table the thread length comes from. |
| Nominal length L | mm | Nominal length of the bolt or screw: under the head to the end (for countersunk heads, overall). |
| Thread length from the standard table (LT, b or B) | mm | Thread length for the size and length from the standard’s table: LT for socket head cap screws (b in BS 4168 and ISO 4762), B for metric bolts and screws, LT for structural bolts. |
| Thread pitch | mm | Pitch of the thread (coarse series unless the fastener is fine pitch). |
| Nominal diameter | mm | Nominal diameter of the fastener; for structural bolts it decides the full-thread rule at 1 in (25.4 mm). |
| Maximum transition thread length Y (structural bolts) | mm | Maximum transition thread length Y for structural bolts from the standard’s table; not used for the other kinds. |
| Joint | ||
| Joint type | choice | Whether the fastener screws into a tapped hole or passes through and takes a nut. |
| Clamped thickness (grip, including washers) | mm | Total thickness of the parts and washers clamped under the head, up to the start of the tapped hole or the nut face. |
| Length needed beyond the joint (thread engagement, or nut height plus projection) | mm | For a tapped hole, the thread engagement the joint needs; for a nut, the nut height plus the projection you want beyond it. |
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: Fastener
- What it is
- Kind of fastener, which sets the thread length rule and the table the thread length comes from.
- Units and format
- Choose one of 3 options from the list: Socket head cap screw (LT, or b for BS 4168 / ISO 4762); Metric bolt or screw (basic thread length B); Heavy hex structural bolt (LT and Y max).
- Example
- The form opens with “Socket head cap screw (LT, or b for BS 4168 / ISO 4762)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Changes the body length rule and the full-thread check.
- Get it right
- Socket head cap screws use LT (b in BS 4168 and ISO 4762); other metric bolts use B; structural bolts use LT and Y.
What this means: Nominal length L
- What it is
- Nominal length of the bolt or screw, from under the head to the end of the shank.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer fastener moves the grip length and the end further out.
- Get it right
- Countersunk screws are measured overall, including the head.
What this means: Thread length from the standard table (LT, b or…
- What it is
- Thread length for this size and length from the fastener standard’s table.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 28 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More thread shortens the grip length and the body.
- Get it right
- It often steps up for lengths over 125 mm and 200 mm; read the right column.
What this means: Thread pitch
- What it is
- Pitch of the thread, the distance from one thread crest to the next.
- 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.
- Effect on the result
- Sets the thread runout of 5P on socket screws and the full-thread limit.
- Get it right
- Coarse pitch unless the fastener is ordered fine pitch; M10 coarse is 1.5 mm.
What this means: Nominal diameter
- What it is
- Nominal diameter of the bolt or screw thread.
- 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.
- Effect on the result
- Only changes the structural bolt full-thread limit.
- Get it right
- For structural bolts it decides whether the full-thread limit is 2.5 or 3.5 pitches.
What this means: Maximum transition thread length Y (structural…
- What it is
- Maximum transition thread length for structural bolts, from the standard’s table.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8.6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer transition shortens the minimum body length.
- Get it right
- Not used for socket screws or other metric bolts.
Joint
What this means: Joint type
- What it is
- Whether the fastener screws into a tapped hole or passes through and takes a nut.
- Units and format
- Choose one of 2 options from the list: Screwed into a tapped hole; Through bolt with a nut.
- Example
- The form opens with “Screwed into a tapped hole” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Changes the wording of the length check beyond the joint.
- Get it right
- Choose a nut for through bolts even when a lock nut is used.
What this means: Clamped thickness (grip, including washers)
- What it is
- Total thickness of the parts and washers clamped under the head.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Must be at least the grip length for the full thread to start within the joint.
- Get it right
- Measure to the start of the tapped thread, or to the nut face for a through bolt.
What this means: Length needed beyond the joint (thread engageme…
- What it is
- Length needed beyond the clamped parts: thread engagement for a tapped hole, or nut height plus projection.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Fails the length when less is left beyond the joint.
- Get it right
- About 1 to 1.5 diameters of engagement in steel and more in aluminium; a nut needs its height plus two threads.
Method and formulas
- Metric bolts and screws: Lg max = L − B.
- Socket head cap screws: LG = L − LT; LTT = LT + 5P; LB = L − LTT (BS 4168: Lg max = L − b, Ls min = Lg max − 5P).
- Structural bolts: LG max = L − LT; LB min = LG max − Y max; threaded full length when LB min ≤ 2.5P (d ≤ 1 in) or 3.5P (d > 1 in).
- Joint: the grip gauging length is the minimum design grip, so the clamped thickness should be at least LG for the nut or tapped hole to work on full thread; length beyond the joint L − grip against the length needed.
- Example: a 40 mm screw with 28 mm of thread has a grip length of 12 mm and leaves 25 mm beyond a 15 mm joint.
Limitations
- Thread lengths LT, b, B and Y are reference dimensions from the fastener standards’ tables; enter them for the size and length.
- Screws too short for the tabulated thread length are threaded close to the head; check the standard for the full-thread lengths of short screws.
- The check does not cover thread strength or engagement length for the materials (use the thread engagement tools).
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a bolt or screw length for a joint
- Check that a screw will not bottom on its thread runout before it clamps
- Inspect grip and body lengths of delivered bolts
Who it is for
- Design engineers
- Fitters and assemblers
- Quality inspectors
What you get
- Grip gauging length (end of full thread from under the head)
- Body or shank length
- Total thread length (socket screws)
- Full-thread status (structural bolts)
- Length beyond the joint against the length needed
- Whether full thread starts within the clamped parts
Interpreting the result
- If the clamped thickness is less than the grip length, the nut or hole meets incomplete thread before the joint is tight: choose a shorter screw or add a washer.
- Shear joints that need the plain shank in the shear plane use the body length, not the grip length.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1572–1573, 1593–1594, 1682, 1696. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Method explained here · KEVOS® 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.
Useful alongside · Library collection
Choosing fasteners and threads: thread systems, grades, screws for sheet and plastics, inserts and pins
In Machine Elements and Mechanical Design
What it covers: Fasteners are cheap, numerous and easy to get wrong. How to identify thread systems, read grades, choose screws, inserts and pins, and cut fastener variety without cutting reliability.
KEVOS® 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.