---
title: "Reverse weld collar"
id: "9340"
type: "guides"
slug: "reverse-weld-collar"
published_at: "2026-05-07T16:12:34+00:00"
modified_at: "2026-05-18T14:11:47+00:00"
url: "https://bighead.co.uk/the-techub/guides/reverse-weld-collar/"
markdown_url: "https://bighead.co.uk/the-techub/guides/reverse-weld-collar.md"
excerpt: "Guidance on specifying reverse weld collar customisation for bigHead fasteners, covering thread sizes, collar lengths and key design considerations."
taxonomy_guide_category:
  - "Customisation guides"
---

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 › Reverse weld collar

Customisation guides

# Reverse weld collar

[Back to guides](../)

## Key takeaways

- You can request a reverse weld collar customisation for internally threaded collar (F2/SF2) bigHead fasteners.
- This customisation flips the weld orientation.
- Useful for high axial loads, fail-safe or safety-critical applications, or for achieving a smooth Head surface.
- Minimum collar length: 10 mm.
- Loadability limits differ from standard collars and are available on request.
- Identified by product code suffix **REV**.

Key dimensions for bigHead with reverse weld collar customisation​

General view of F2/S30 bigHead with reverse weld collar customisation (SF2/S30 is similar)

Section view of F2/S30 bigHead with reverse weld collar customisation (SF2/S30 is similar)

## Customisation scope for reverse weld collar

Reverse weld collars have a flipped weld orientation. The collar shank passes through the sighted Head, and the weld is applied to the flange surface on the shank side.

The collar fixing and Head geometries remain standard.

This customisation applies to [sighted internally threaded collar fasteners (F2 and SF2)](https://bighead.co.uk/bighead-products/sighted-internally-threaded-collar/s20/)
.

### **Compatible thread sizes by Head type**

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

### **Reverse weld collar lengths by thread size**

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

Due to manufacturing constraints, collar lengths <10 mm are not possible.

## What's outside the scope of reverse weld collar customisation

Some requests cannot be combined with reverse weld collar customisation, due to manufacturing constraints. Examples:

- Collar lengths below 10 mm
- Non‑standard thread lengths
- Changes to collar and Head geometry

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 reverse weld collar

### Product length

With reverse weld collars, the collar length (L) becomes the overall product length. This differs from standard collar products, where overall length includes both collar length and Head thickness.

### Usable thread length

Screw engagement within the internal thread is approximately equal to the collar length.

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)
.

### Loadability

Load conditions and loadability limits are different to standard products, due to the reversed load path. We can provide this information on request.

## Specification examples for reverse weld collar customisation

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

To specify a reverse weld collar customisation, M6 × 20 mm, on a S20 head:

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

**→ F2/S20 M6 x 20 REV**

- Usable thread length (Le): 20 mm
- Overall product length: 20 mm

### Example 2: 316 stainless steel

To specify a reverse weld 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: REV

**→ SF2/B38A M10 × 15 REV**

- Usable thread length (Le): 15 mm
- Overall product length: 15 mm

### Example 3: Out of scope

A request for M4 × 5 mm reverse weld 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 reverse weld 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 reverse weld 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 does reverse weld collar customisation involve? It reorients the Head position, so that the collar shank passes through the sighted Head. The weld is applied to the flange on the shank side, instead of the Head being welded to the flange end.
- When do I need reverse weld collar customisation? Reverse weld collars are useful for applications where: - Axial fastening loads or tensile stresses may exceed the weld strength of a standard collar, or - You need to present a smooth, uninterrupted Head surface for component seating or moulding shut‑off.
- Which bigHead products are compatible with reverse weld collar? This customisation applies only to sighted internally threaded collar types (F2/SF2) and specific Head types including S20, S23, ST38A, S30, S32 and S38A, with defined thread‑size ranges for each.
- How does the reverse weld affect overall product length and screw engagement? With reverse weld collar, the overall product length is the same as the collar length. This differs from standard collar products, where overall length includes both collar length and Head thickness. Screw engagement is approximately equal to the full collar length.
- Are load limits the same as standard F2/SF2 collars? No. The reverse weld orientation creates different load paths. We can provide load-limit values and tightening torque recommendations on request to support correct application.

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