---
title: "Anti-loosening/lock nuts"
id: "9332"
type: "guides"
slug: "anti-loosening-lock-nuts"
published_at: "2026-05-07T14:47:34+00:00"
modified_at: "2026-05-18T14:10:04+00:00"
url: "https://bighead.co.uk/the-techub/guides/anti-loosening-lock-nuts/"
markdown_url: "https://bighead.co.uk/the-techub/guides/anti-loosening-lock-nuts.md"
excerpt: "Guidance on specifying anti‑loosening lock‑nut customisation for bigHead fasteners, including thread sizes, compatibility and performance notes."
taxonomy_guide_category:
  - "Customisation guides"
---

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 › Anti-loosening/lock nuts

Customisation guides

# Anti-loosening/lock nuts

[Back to guides](../)

## Key takeaways

- You can request anti-loosening customisation for sighted internally threaded nut (F1/SF1) bigHead fasteners.
- This customisation replaces the standard nut with an all-metal prevailing‑torque lock nut.
- It provides anti‑loosening functionality under vibration by resisting rotation of the nut body.
- Available for M6, M8, M10, M12 thread sizes only. Requests outside this scope require our Create service.
- Compatible with standard sighted Head types.
- Running torque is higher than standard nuts.
- Lock‑nut geometry differs from standard fasteners (height, thread length, across‑flats).
- Identified by product code suffix **PTS**.

Key dimensions for bigHead with anti-loosening/lock nut customisation

General view of F1/S30 bigHead with anti-loosening/lock nut customisation (SF1/S30 is similar)

Section view of F1/S30 bigHead with anti-loosening/lock nut customisation (SF1/S30 is similar)

## Customisation scope for anti-loosening/lock nuts

This customisation replaces the standard F1/FS1 nut with a fixed‑design, two‑piece, all‑metal prevailing‑torque lock nut.

Due to the lock nut design, it is available for M6, M8, M10 and M12 thread sizes only.

### Head type options

Head options follow [standard F1/SF1 availability](https://bighead.co.uk/bighead-products/sighted-internally-threaded-nut/s20/)
. The Head type options, by thread size, are:

- M6 and M8 → S20, S23, S30, S32, S38A, ST38A
- M10 and M12 → S30, S32, S38A

### Nut body materials

For carbon steel bigHead products, the nut body is grade 8 carbon steel. For stainless steel bigHead products, the nut body is 316 stainless steel.

### Finishes

Standard bigHead finishes apply (BZP for carbon steel, natural finish for 316 stainless steel).

## What's outside the scope of anti-loosening customisation

Some requests are not compatible with anti-loosening customisation, due to manufacturing constraints. Examples:

- M4 or M5 thread sizes
- Alternative Head designs
- Alternative nut forms (flange, reduced height)
- Alternative stainless grades
- DIN 980 variants
- Non‑standard coatings

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 anti-loosening/lock nuts

### Geometry differences

Compared to standard F1/ SF1 products, the key geometric differences are overall nut height (L), thread length (Le), and across flats distance (D2).

Values for these dimensions, by thread size, are:

- M6: L = 5.1 mm, Le = 4.2 mm, D2 = 10 mm
- M8: L = 7.3 mm, Le = 6.1 mm, D2 = 13 mm
- M10: L = 8.3 mm, Le = 7.1 mm, D2 = 17 mm
- M12: L = 10.5 mm, Le = 9 mm, D2 = 19 mm

### Design and performance notes

- Running torque (not tightening torque) for screw installation is higher due to the prevailing-torque feature.
- You may need to use a longer screw.
- Weld load limit (WLL) and tightening torque values differ from standard products. We can provide these values on request.

If you’re not sure how this affects your assembly, [get in touch](https://bighead.co.uk/?post_type=guides&p=9329&preview=true#contact-form)
.

## Specification examples for anti-loosening customisation

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

To specify anti-loosening customisation on a round S20 Head with M6 thread size:

- Product type: F1/ S20
- Thread diameter: M6
- Anti-loosening/ lock nut customisation suffix: PTS

**→ F1/S20 M6 PTS**

### Example 2: 316 stainless Steel

To specify anti-loosening customisation on a square S32 Head with M10 thread size:

- Material prefix: S (316 stainless steel)
- Product type: F1/ S32
- Thread diameter: M10
- Anti-loosening/ lock nut customisation suffix: PTS

**→ SF1/S32 M10 PTS**

### Example 3: Out of scope

A request for anti-loosening customisation of an SF1/B20 M4 product is not possible through our customisation service. Instead, we can create a solution for you through our [Create service](https://bighead.co.uk/bespoke/create/)
.

# Next steps

## Requesting an anti-loosening customisation

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

If needed, we can help you define whether anti-loosening customisation 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 the anti-loosening customisation actually change? It replaces the standard F1/SF1 nut with a prevailing–torque lock nut, which incorporates a stainless steel locking element that resists rotation under vibration.
- Will the mechanical performance change compared to standard bigHead F1/SF1 products? Yes. The lock nut design uses different geometry, and the associated WLL and tightening torque values are not the same as standard products. We can supply the correct values for your specification on request.
- Why is installation torque higher than with standard nuts? Prevailing–torque lock nuts introduce additional friction via a mechanical locking feature, increasing the running torque required during assembly.
- Which thread sizes can be specified with anti-loosening customisation? M6, M8, M10, and M12 only. M4 and M5 are unavailable through this route due to our product manufacturing limitations. Contact us to ask about alternative options for M4 or M5 thread sizes.
- Will I need a different screw length? Possibly. The lock nut design has different height and thread engagement lengths, so you may need a slightly longer screw to ensure proper engagement with the locking feature.

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