---
title: "Closed blind collar"
id: "9334"
type: "guides"
slug: "closed-blind-collar"
published_at: "2026-05-07T15:09:26+00:00"
modified_at: "2026-05-18T14:10:24+00:00"
url: "https://bighead.co.uk/the-techub/guides/closed-blind-collar/"
markdown_url: "https://bighead.co.uk/the-techub/guides/closed-blind-collar.md"
excerpt: "Guidance on specifying closed blind collar customisation for bigHead fasteners, including thread sizes, collar lengths and key design considerations."
taxonomy_guide_category:
  - "Customisation guides"
---

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 › Closed blind collar

Customisation guides

# Closed blind collar

[Back to guides](../)

## Key takeaways

- You can request a closed blind collar customisation for blind internally threaded collar (F2/SF2) bigHead fasteners.
- This customisation replaces the standard open collar with a blind flange collar to prevent resin or polymer 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).
- Fixing load limits and recommended tightening torques match standard F2/SF2 collar fasteners.
- Weld load limits may differ. We can supply these on request.
- Identified by product code suffix **BCL**.

Key dimensions for bigHead with closed blind collar customisation

General view of F2/B30 bigHead with closed blind collar customisation (SF2/B30 is similar)

Section view of F2/B30 bigHead with closed blind collar customisation (SF2/B30 is similar)

## Customisation scope for closed blind collar

Closed blind collars pair a blind Head with a blind flange collar. This customisation applies to [blind internally threaded collar fasteners (F2 and SF2)](https://bighead.co.uk/bighead-products/blind-internally-threaded-collar/b20/)
.

This customisation uses the same collar length increments as our standard blind 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

- B20, B23 and T38A; M4, M5, M6 and M8
- B30, B32, and B38A; M4, M5, M6, M8, M10 and M12

### Closed blind 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 blind 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 blind collar

### Internal geometry and thread acceptance

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

With this customisation, the blind flange feature reduces the maximum screw insertion depth. The unusable length (Lu) for closed blind 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 blind 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) and recommended tightening torque are the same as standard collar fasteners.

Weld load limit (WLL) may differ. We can supply information on request.

## Specification examples for closed blind collar customisation

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

To specify a closed blind collar customisation, M6 × 20 mm, on a B20 Head:

- Product type: F2/B20
- Thread × collar length: M6 × 20
- Customisation suffix: BCL

**→ F2/B20 M6 × 20 BCL**

- Usable thread length (Le): 15 mm

### Example 2: 316 stainless steel

To specify a closed blind collar customisation, M10 × 15 mm, on a B38A Head:

- Material prefix: S (316 stainless steel)
- Product type: F2/B38A
- Thread × collar length: M10 × 5
- Customisation suffix: BCL

**→ SF2/B38A M10 × 15 BCL**

- Usable thread length (Le): 9 mm

### Example 3: Out of scope

A request for M8 5 mm closed blind collar customisation on a T38A 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 blind 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 blind 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 blind collar customisation? The blind flange collar design prevents resin or polymer ingress during moulding or composite processing, protecting the thread from blockage and reducing rework.
- Can all bigHead fasteners come with a closed blind collar? No. This customisation applies only to F2/SF2 internally threaded collars used with blind Head types B20, B23, B30, B32, B38A and T38A.
- What are the compatible standard collar lengths for this customisation? M4: 10 & 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 blind collars? Standard collar fixing load limits (FLL) and torque recommendations apply. Weld load limits (WLL) may differ and are available on request.

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