---
title: "Closed sighted collar"
id: "9335"
type: "guides"
slug: "closed-sighted-collar"
published_at: "2026-05-07T15:20:29+00:00"
modified_at: "2026-05-18T14:10:41+00:00"
url: "https://bighead.co.uk/the-techub/guides/closed-sighted-collar/"
markdown_url: "https://bighead.co.uk/the-techub/guides/closed-sighted-collar.md"
excerpt: "Guidance on specifying closed sighted collar customisation for bigHead fasteners, including thread sizes, collar lengths and key design considerations."
taxonomy_guide_category:
  - "Customisation guides"
---

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 › Closed sighted collar

Customisation guides

# Closed sighted collar

[Back to guides](../)

## Key takeaways

- You can request a closed sighted collar customisation for sighted internally threaded collar (F2/SF2) bigHead fasteners.
- This customisation replaces the standard open collar with a blind shank collar to prevent fluid or moisture ingress.
- Usable thread length (Le) is reduced by 5–7 mm depending on thread size.
- Minimum collar lengths: 10 mm (M4–M8), 15 mm (M10–M12).
- Load limits and tightening torque values match standard collar fasteners.
- Identified by product code suffix **CL**.

Key dimensions for bigHead with closed sighted collar customisation

General view of F2/S30 bigHead with closed sighted collar customisation (SF2/S30 is similar)

Section view of F2/S30 bigHead with closed blind collar customisation (SF2/S30 is similar)

## Customisation scope for closed sighted collar

Closed sighted collar fasteners pair a sighted Head with a blind shank collar. The customisation applies to [sighted internally threaded collar fasteners (F2 and SF2)](https://bighead.co.uk/bighead-products/sighted-internally-threaded-collar/s20/)
.

This customisation uses the same collar length increments as our standard sighted collar fasteners, but with a manufacturing constraint on the minimum collar length.

All other collar geometry – shank diameter, flange outer diameter, and flange ring thickness –matches our standard collar designs.

### Compatible thread sizes by Head type

- S20, S23 and ST38A; M4, M5, M6 and M8
- S30, S32, and S38A; M4, M5, M6, M8, M10 and M12

### Closed sighted collar lengths (L) by thread size

- M4 → 10 and 15 mm
- M5, M6 and M8 → 10, 15 and 20 mm
- M10 and M12 → 15 and 20 mm

### Manufacturing constraints

- Collar lengths below 10 mm are not possible for M4–M8
- Collar lengths below 15 mm are not possible for M10–M12

## What's outside the scope of closed sighted collar customisation

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

- Collar lengths below manufacturing minimums
- Non‑standard thread lengths
- Changes to shank diameter, flange outer diameter, or flange ring thickness

If you have an out-of-scope request, we can work with you to create a solution – [get in touch](https://bighead.co.uk/?post_type=guides&p=9329&preview=true#contact-form)
.

## Design and performance considerations for closed sighted collar

### Internal geometry and thread acceptance

The usable thread length (Le) for bigHead collar products with sighted Heads is measured from the collar flange end‑face.

With this customisation, the blind shank geometry reduces the maximum screw insertion depth. The unusable length (Lu) for closed sighted collar customisations varies according to thread size:

- M4–M8 → 5 mm
- M10 → 6 mm
- M12 → 7 mm

**Usable thread length (Le):**

The usable thread length (Le) determines the maximum screw insertion depth.

*Le = L – Lu*

**Example:***M6 × 20 closed sighted collar, L = 20 mm → Le = 15 mm*

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/?post_type=guides&p=9329&preview=true#contact-form)
.

### Performance

Fixing load limit (FLL), recommended tightening torque, and weld load limit (WLL) are the same as standard collar fasteners.

## Specification examples for closed sighted collar customisation

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

To specify a closed sighted collar customisation, M6 × 20 mm, on a S20 Head:

- Product type: F2/S20
- Thread × collar length: M6 × 20
- Customisation suffix: CL

**→ F2/S20 M6 × 20 CL**

- Usable thread length (Le): 15 mm

### Example 2: 316 stainless steel

To specify a closed sighted collar customisation, M10 × 15 mm, on a S38A Head:

- Material prefix: S (316 stainless steel)
- Product type: F2/S38A
- Thread × collar length: M10 × 15
- Customisation suffix: CL

**→ SF2/B38A M10 × 15 CL**

- Usable thread length (Le): 9 mm

### Example 3: Out of scope

A request for M8 × 5 mm closed sighted collar customisation on a ST38A Head is not possible through our customisation service. A minimum 10 mm collar length is needed for manufacturing feasibility.

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

# Next steps

## Requesting a closed sighted collar customisation

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

If needed, we can help you define whether a closed sighted collar is suitable 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

- What is the main advantage of a closed sighted collar? The blind shank design prevents fluid or moisture ingress, protecting the thread from adverse effects due to environmental exposure.
- Can all bigHead fasteners come with a closed sighted collar? No. This customisation applies only to F2/SF2 internally threaded collars with sighted Head types S20, S23, S30, S32, S38A and ST38A.
- What are the compatible standard collar lengths for this customisation? M4: 10 and 15 mm M5 to M8: 10, 15 and 20 mm M10 and M12: 15 and 20 mm
- How do I calculate the maximum usable thread length (Le)? Subtract the unusable length (Lu) from the collar length (L): Le = L – Lu.
- Do load limits or torque settings change for closed sighted collars? No. Standard collar fixing load limits (FLL and WLL) and torque recommendations apply.

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