--- title: "PTFE USIT Rings (Self-Centring Bonded Seals)" description: "PTFE USIT Rings (Self-Centring Bonded Seals) (polytetrafluoroethylene (thermoplastic, not an elastomer)): near-universal chemical resistance; no elastic" url: https://engineeringhardware.com/guide/material/ptfe-usit-rings/ source: Engineering Hardware & Fasteners --- # PTFE USIT Rings (Self-Centring Bonded Seals) Seals · Trade series (self-centring bonded washers, USIT type) Temperature range, chemical and fluid compatibility, hardness and the full dimensional range for this compound. PTFE USIT Rings (Self-Centring Bonded Seals) are supplied to polytetrafluoroethylene (thermoplastic, not an elastomer). Corrosion behaviour is near-universal chemical resistance; no elastic recovery, so it is used as a back-up or energised seal, the material is non-magnetic, and the usable service range is approx. -200 °C to +260 °C. In practice this grade is chosen for chemical service and low friction rather than for elastic sealing. - **Grade:** polytetrafluoroethylene (thermoplastic, not an elastomer) - **Corrosion resistance:** Near-universal chemical resistance; no elastic recovery, so it is used as a back-up or energised seal - **Magnetic response:** Non-magnetic - **Service temperature:** approx. -200 °C to +260 °C Property values are typical for the grade. Certified mechanical test data is available on request for production orders. ![PTFE USIT Rings (Self-Centring Bonded Seals) reference image](https://engineeringhardware.com/wp-content/uploads/2026/09/USIT-Rings.webp) Material page for usit rings (self-centring bonded seals) in PTFE (polytetrafluoroethylene (thermoplastic, not an elastomer)). The sizes below are the ones we supply; there is no published dimensional standard for this article, so the table rather than a standard number is what to quote. Corrosion behaviour is near-universal chemical resistance; no elastic recovery, so it is used as a back-up or energised seal and the service range is approx. -200 °C to +260 °C. ## Why Choose PTFE for USIT Rings (Self-Centring Bonded Seals) Chosen for chemical service and low friction rather than for elastic sealing. For usit rings (self-centring bonded seals), this matters when near-universal chemical resistance; no elastic recovery, so it is used as a back-up or energised seal is the primary requirement. ## Mechanical & Physical Properties | Grade designation | polytetrafluoroethylene (thermoplastic, not an elastomer) | | --- | --- | | Magnetic response | Non-magnetic | | Service temperature | approx. -200 °C to +260 °C | | Corrosion characteristic | Near-universal chemical resistance; no elastic recovery, so it is used as a back-up or energised seal | ## Corrosion Behavior in Service Carbon and alloy steel usit rings (self-centring bonded seals) require a coating for corrosion protection. Zinc plating (with or without passivation) is the most common; hot-dip galvanizing is used for outdoor structural service; black-oxide is mainly a tarnish-prevention finish for tool-room use. ## Comparable Grades For usit rings (self-centring bonded seals), consider an energised or spring-loaded PTFE seal where recovery is required, or FKM where the chemistry allows an elastomer. Elastomer selection is decided by the medium, the temperature range and whether the seal is static or dynamic — not by strength. ## Sizes of USIT Rings (Self-Centring Bonded Seals) Available in PTFE | 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 | Dimensions are independent of the material: every size above is available in PTFE. Custom manufacturing · Made to drawing ## Need this seal in a grade that is not listed? Ptfe is one of the grades the catalog describes. 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 M6 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 inner lip - **Metal carrier:** zinc-plated steel - **Typical service:** hydraulic ports and plugs ## 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 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's the service temperature limit for PTFE? The practical service range is approx. -200 °C to +260 °C. ### Are PTFE USIT Rings (Self-Centring Bonded Seals) magnetic? PTFE USIT Rings (Self-Centring Bonded Seals) are non-magnetic. ### What corrosion resistance do PTFE USIT Rings (Self-Centring Bonded Seals) offer? PTFE USIT Rings (Self-Centring Bonded Seals) provide near-universal chemical resistance; no elastic recovery, so it is used as a back-up or energised seal with a service range of approx. -200 °C to +260 °C. ### Which media is PTFE suitable for? PTFE suits almost every chemical, over a very wide temperature range. It is the wrong choice for anything needing elastic recovery — it cold-flows and takes a set. Elastomer selection is decided by the fluid first, then the temperature, then whether the seal is static or dynamic. ### 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