---
title: "Thread length variants"
id: "9338"
type: "guides"
slug: "thread-length-variants"
published_at: "2026-05-07T15:58:41+00:00"
modified_at: "2026-05-18T14:12:04+00:00"
url: "https://bighead.co.uk/the-techub/guides/thread-length-variants/"
markdown_url: "https://bighead.co.uk/the-techub/guides/thread-length-variants.md"
excerpt: "Guidance on specifying thread‑length variants for bigHead studs and collars, covering ranges, compatibility and key design considerations."
taxonomy_guide_category:
  - "Customisation guides"
---

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 › Thread length variants

Customisation guides

# Thread length variants

[Back to guides](../)

## Key takeaways

- Thread length can be customised in 1 mm increments for both studs (M1/SM1) and collars (F2/SF2).
- This customisation gives you precise control over thread engagement, and accommodates unique assembly stack-ups and access constraints.
- Studs are available in custom thread lengths of 5 – 100 mm (depending on thread size).
- Collars are available in thread lengths of 5 – 40 mm (M4–M10) and 10 – 40 mm (M12).
- All other geometry remains standard – only the threaded section length changes.
- Blind collars require screw tip clearance, which reduces usable thread length.
- Performance and loadability remain the same as standard bigHead products.

## Customisation scope for thread length variants

### Externally threaded studs (M1/SM1)

Thread length (L) can be customised in 1 mm increments, within these ranges according to thread size:

- M4: 5 – 40 mm
- M5: 5 – 60 mm
- M6: 10 – 100 mm
- M8: 12 – 100 mm
- M10: 20 – 100 mm

Shoulder geometry remains the same as the equivalent standard stud.

**Compatible thread sizes by Head type:**

- B15: M4 – M6
- B20, B23: M4 – M8
- B30, B32, B38A, T38A: M4 – M10

Key dimensions for stud bigHead with thread length customisation

General view of M1/B30 bigHead with thread length customisation (SM1/B30 is similar)

### Internally threaded collars (F2/SF2)

Collar lengths can be customised in 1 mm increments, within these ranges according to thread size:

- M4 – M10: 5 – 40 mm
- M12: 10 – 40 mm

All other collar geometry remains standard.

**Compatible thread sizes by Head type:**

Blind Heads (screw does not pass through):

- B20, B23, T38A: M4 – M8
- B30, B32, B38A: M4 – M12

Sighted Heads (screw passes through):

- S20, S23, ST38A: M4 – M8
- S30, S32, S38A: M4 – M12

Key dimensions for blind collar bigHead with thread length customisation

General view of F2/B30 bigHead with thread length customisation (SF2/B30 is similar)

Section view of F2/B30 bigHead with thread length customisation (SF2/B30 is similar)

Key dimensions for sighted collar bigHead with thread length customisation

General view of F2/S30 bigHead with thread length customisation (SF2/S30 is similar)

Section view of F2/S30 bigHead with thread length customisation (SF2/S30 is similar)

## What's outside the scope of thread length customisation

Some requests are out-of-scope due to manufacturing constraints. Examples:

- Thread lengths outside the minimum and maximum values
- Changes to collar and Head geometry
- Combining thread length variants with other customisations (e.g. alternative finishes)

If you have an out-of-scope request, we can work with you to create a solution – [get in touch](https://bighead.co.uk/the-techub/guides/large-round-head/#contact-form)
.

## Design and performance considerations for thread length variants

Thread‑length customisation affects only the threaded element of the stud or collar. All other geometry, and therefore all loadability characteristics, remain unchanged.

The main design considerations relate to overall product length, screw engagement, and screw clearance requirements.

### Externally threaded studs (M1/SM1)

Custom thread length (L) determines the majority of the overall product length. Two additional dimensions also contribute:

- Shoulder height (L1):
  - M4 – M6 → 1.2 mm
  - M8 – M10 → 1.5 mm

- Head thickness (T): 1.2 mm

Overall length formula: *Total length = L + L1 + T*

#### Examples

Minimum overall lengths:

- M4 and M5 = 7.4 mm (5 + 1.2 + 1.2)
- M6 = 12.4 mm (10 + 1.2 + 1.2)
- M8 = 14.7 mm (12 + 1.5 + 1.2)
- M10 = 22.7 mm (20 + 1.5 + 1.2)

Maximum overall lengths:

- M4 = 42.4 mm (40 + 1.2 + 1.2)
- M5 = 62.4 mm (60 + 1.2 + 1.2)
- M6 = 102.4 mm (100 + 1.2 + 1.2)
- M8 and M10 = 102.7 mm (100 + 1.5 + 1.2)

These calculations confirm the overall stud length, on which you can base the necessary clearances to prevent clashes with nearby components during assembly or with mould tooling during co‑processing.

### Internally threaded collars (F2/SF2)

For collars, the custom length (L) determines both:

- Overall product length, and
- Maximum screw engagement (with different rules for blind vs sighted Heads)

Overall length formula: *Total = L + head thickness (1.2 mm)*

#### Examples:

- M4 – M10 minimum: 6.2 mm
- M12 minimum: 11.2 mm
- All sizes maximum: 41.2 mm

#### Blind Heads – screw tip clearance

Blind collars require a clearance allowance (Lu) to prevent the screw tip from bottoming out, which can cause deformation and irreparable damage:

- M4: 1.4 mm
- M5: 1.6 mm
- M6: 2 mm
- M8: 2.5 mm
- M10: 3 mm
- M12: 3.5 mm

Usable thread length: *Le=L-Lu*

This value determines the maximum screw thread acceptance for blind Head collars.

#### Sighted Heads – screw engagement

For sighted collars, the screw passes through the Head. Therefore:

- Maximum screw engagement ≈ collar length (L)
- No clearance deduction is required

For help, see our [guide to screw length selection](https://bighead.co.uk/the-techub/guides/selecting-screw-lengths-for-bighead-collar-fasteners/)
, or [get in touch](https://bighead.co.uk/the-techub/guides/selecting-screw-lengths-for-bighead-collar-fasteners/#contact-form)
.

## Specification examples for thread length customisation

### Example 1: Carbon steel, bright zinc plate (BZP) finish

To specify an externally threaded stud, M5 × 50 mm, on a B20 Head:

- Product type: M1/B20
- Thread diameter: M5
- Customised thread length (L): 50 mm
- Stud material suffix: HT (carbon steel, heat treated)

**→ M1/B20 M5 × 50 HT**

### Example 2: 316 stainless steel

To specify an internally threaded collar, M8 × 32 mm, on a sighted S38A Head:

- Material prefix: S (316 stainless steel)
- Product type: F2/S38A
- Thread diameter: M8
- Customised collar length (L): 32 mm

**→ SF2/S38A M8 × 32**

### Example 3: Out of scope

A request for M10 × 50 mm collar on a blind T38A head is not possible through our customisation service. 50 mm exceeds the 40 mm maximum collar length and M10 thread size is outside the T38A Head compatibility scope.

Instead, we can create a solution for you through our [Create service](https://bighead.co.uk/bespoke/create/)
.

# Next steps

## Requesting a thread length customisation

Contact us with your required product code to request a quote or confirm suitability.

If needed, we can help you determine the correct thread length for your application.

**Contact:**  
 Email: [sales@bigHead.co.uk](mailto:sales@bigHead.co.uk)
  
 Tel: +44 (0) 1202 574601

**Sales availability:**  
 Mon–Thu: 07:30–16:15 (UK)  
 Fri: 07:30–12:15 (UK)

[Get in touch](#contact-form)

## FAQs

- Can I request a thread length outside the minimum or maximum values? No. These limits reflect manufacturing capability. If your design requires something different, we can help you find the closest viable alternative.
- Does changing the thread length affect the performance of the bigHead? No. Loadability and material properties remain unchanged. A longer or shorter thread only affects the screw engagement you can achieve during assembly.
- Are there different tolerances for custom thread lengths? No. Custom thread lengths follow the same dimensional tolerances as our standard stud and collar products. This ensures consistent fit, function, and installation performance.
- How do I select the correct screw length for an internally threaded collar? For sighted collars: screw engagement ≈ collar length. For blind collars: allow clearance (Lu) to avoid screw‑end contact. So Le = L – Lu If you’re unsure, see our [guide to screw thread length selection](https://bighead.co.uk/the-techub/guides/selecting-screw-lengths-for-bighead-collar-fasteners/) , or get in touch.
- Does custom thread length affect lead time or minimum order quantities? Depending on the length selected, it may do. We’ll confirm this when quoting your specification.

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