---
title: "M18 USIT Rings (Self-Centring Bonded Seals) Dimensions"
description: "M18 USIT Rings (Self-Centring Bonded Seals): coarse thread pitch 2.5 mm."
url: https://engineeringhardware.com/guide/size/m18-usit-rings/
source: Engineering Hardware & Fasteners
---

# M18 USIT Rings (Self-Centring Bonded Seals) Dimensions

Seals · Trade series (self-centring bonded washers, USIT type)

Controlling dimensions to Trade series (self-centring bonded washers, USIT type), with tolerances and fit.

M18 USIT Rings (Self-Centring Bonded Seals) dimensions to Trade series (self-centring bonded washers, USIT type): coarse thread pitch 2.5 mm.

- **Coarse thread pitch:** 2.5 mm

Figures are nominal values from the governing standard. Confirm against the tolerance class for your application before machining or ordering.

![M18 USIT Rings (Self-Centring Bonded Seals) Dimensions reference image](https://engineeringhardware.com/wp-content/uploads/2026/09/USIT-Rings.webp)

M18 USIT Rings (Self-Centring Bonded Seals) are manufactured to Trade series (self-centring bonded washers, USIT type). This page lists the controlling dimensions for the M18 size together with the installation notes for this type.

Values are taken from the governing standard rather than from a general machine-thread table, because this type is not selected on one.

## M18 Dimensional Row (Trade series (self-centring bonded washers, USIT type))

| Thread | Seal element | Metal carrier | Typical service |
| --- | --- | --- | --- |
| M18 | NBR or FKM inner lip | zinc-plated or stainless | hydraulic |

Custom manufacturing · Made to drawing

## Need USIT rings (self-centring bonded seals) in a size this table does not list?

This page covers M18 USIT rings (self-centring bonded seals). A special seal is almost always a compound change rather than a size change — the same dimensions in a material that survives the medium.

Minimum order from 2,000 pieces; samples within 7 days of drawing approval.

[Custom seal manufacturing](https://engineeringhardware.com/custom/seals/)[Send a drawing](https://engineeringhardware.com/custom/seals/#rfq)

## Technical Drawing

Dimensioned line drawing of M18 USIT Rings (Self-Centring Bonded Seals) to Trade series (self-centring bonded washers, USIT type). Side view and top view, drawn to the dimensions in the table below and redrawn for any size selected.

- **Seal element:** NBR or FKM inner lip
- **Metal carrier:** zinc-plated or stainless
- **Typical service:** hydraulic

## Dimensions & Specifications (Trade series (self-centring bonded washers, USIT type))

| Thread | Seal element | Metal carrier | Typical service |
| --- | --- | --- | --- |
| M6 | NBR inner lip | zinc-plated steel | hydraulic ports and plugs |
| M8 | NBR inner lip | zinc-plated steel | hydraulic ports and plugs |
| M10 | NBR inner lip | zinc-plated steel | hydraulic ports and plugs |
| M12 | NBR inner lip | zinc-plated steel | hydraulic ports and plugs |
| M14 | NBR or FKM inner lip | zinc-plated or stainless | hydraulic and fuel |
| M16 | NBR or FKM inner lip | zinc-plated or stainless | hydraulic and fuel |
| M18 | NBR or FKM inner lip | zinc-plated or stainless | hydraulic |
| M20 | NBR or FKM inner lip | zinc-plated or stainless | hydraulic |
| M22 | NBR inner lip | zinc-plated steel | hydraulic |
| M27 | NBR inner lip | zinc-plated steel | hydraulic |

Product catalog

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## Applications & Industries for M18 USIT Rings

USIT Rings are bought for a small number of well-defined jobs. These are the ones we ship them for. Stocked from M6 to M27.

### Industries that specify USIT Rings

- **Machine tools:** Coolant and hydraulic ports.
- **Hydraulics:** Port plugs and fittings.
- **Mobile plant:** Vibration-prone joints.
- **Compressors:** Sealing under pressure cycling.
- **Process & food equipment:** Sealing at joints that are cleaned and inspected.
- **Hydraulics & pneumatics:** Static and dynamic sealing in cylinders and valve bodies.

### Typical applications

- seating the ring square before tightening
- choosing the self-centring form where the seat is imperfect
- matching the thread size exactly
- selecting FKM where the fluid or temperature demands it
- excluding dust and moisture from a housing
- sealing a joint against fluid or air pressure

### Service conditions to specify against

- **Sealed joints:** Used where the fixing has to keep water or air out as well as hold — the elastomer, not the fastener, decides service life.

### Where to use something else

- Do not reuse a bonded or USIT seal after it has been compressed; the elastomer has taken a set and will not seal a second time.

[Full application guide: where USIT Rings are used, how to specify them, and the cases they are wrong for](https://engineeringhardware.com/applications/usit-rings/)

Inside the plant

## Manufactured in our own plant

This part is cold-formed, threaded, heat-treated and finished in-house. The stages below are where its class and finish are actually decided.

1. ![Wire rod feeding into a cold heading machine](https://engineeringhardware.com/wp-content/uploads/2026/08/09-wire-rod-feed-cold-heading-machine.webp) 01 **Cold heading** Wire rod is drawn to size and cold-formed into blanks. Forming rather than cutting keeps the grain flow continuous around the head, which is where a bolt is most likely to fail. Head dimensions and under-head radius checked at set-up and per shift.
2. ![Thread rolling machine producing bolt threads](https://engineeringhardware.com/wp-content/uploads/2026/08/03-thread-rolling-machine-bolt-production.webp) 02 **Thread rolling** Threads are rolled between hardened dies rather than cut. Rolled threads are stronger in fatigue than cut threads and hold pitch and flank angle far more consistently across a batch. Go / no-go ring gauges to ISO 965 tolerance class 6g.
3. ![Baskets of bolts staged for heat treatment](https://engineeringhardware.com/wp-content/uploads/2026/08/05-heat-treatment-baskets-black-bolts.webp) 03 **Heat treatment** Quench-and-temper brings carbon and alloy steel to the property class ordered — 8.8, 10.9 or 12.9 — with the furnace charge tracked so a finished lot can be traced back to it. Core hardness and tensile tested per lot to ISO 898-1.
4. ![Zinc plated bolts in a treatment basket](https://engineeringhardware.com/wp-content/uploads/2026/08/23-zinc-plated-bolts-treatment-basket.webp) 04 **Surface treatment** Zinc, zinc-nickel, hot-dip galvanizing, black oxide, phosphate and Dacromet, selected against the service environment rather than against the price list. Coating thickness and neutral salt spray hours to ISO 9227.

## Frequently Asked Questions

### What grip range is typical for M18 structural bolts?

Grip ranges are highly application-dependent at M18. As a guide, choose a length that gives at least 1× the thread diameter engagement in the nut plus 2–3 thread pitches projecting.

### What is the coarse-thread pitch of M18 for USIT Rings (Self-Centring Bonded Seals)?

For M18 the standard coarse pitch is 2.5 mm — this is the value used for USIT Rings (Self-Centring Bonded Seals).

### What clearance and tapping drill suit M18?

Use a 20 mm clearance hole (ISO 273 medium) for through-fixing, or a 15.5 mm tapping drill for a coarse M18 thread.

### Should M18 USIT Rings (Self-Centring Bonded Seals) be tightened by torque or by angle?

For critical structural joints at M18, angle-controlled or stretch-controlled tightening is more repeatable than pure torque — the friction scatter at large diameters can dominate the torque signal.

### How do I request a quotation?

Use the technical drawing on this page to pick the exact configuration, then submit it with the quote form. We respond within one business day.

## Related Pages

[View the full USIT Rings (Self-Centring Bonded Seals) page](https://engineeringhardware.com/fastener/usit-rings/)

### More USIT Rings (Self-Centring Bonded Seals) Guides