PN/ČSN 02 9281: which O-ring replaces this old standard?

You are holding an old drawing or a worn part that calls out a ring to 02 9281 — sometimes written PN 02 9281, ČSN 02 9281 or the Slovak twin STN 02 9281 — and you need to know two things: what it actually specifies, and which O-ring you order today. This article answers both. The short version: 02 9281 is a legacy Czechoslovak national standard for static-sealing O-rings, it was cancelled in 1997 without replacement, and the safe way to source a part is to read the real ring in millimetres rather than trust the old code.

What PN/ČSN 02 9281 is

02 9281 is a Czechoslovak national standard for O-rings used to seal non-moving parts — flanges, covers, housings and similar static joints. The Slovak version STN 02 9281 was issued on 1 May 1960 and the ČSN edition has been valid since 1961. It belongs to a small family of related standards from the same era: PN 02 9280 covers dynamic (moving) sealing, while the sibling PN 02 9283 covers the modern metric series. Rubena, the original Czechoslovak O-ring maker, still groups its rings into exactly these size series — DIN 3770, PN 02 9280, PN 02 9281 and PN 02 9283.

The standard was cancelled in December 1997 without a successor. That is the single most important fact for a buyer: there is no current norm that carries the 02 9281 designation forward. It survives only as a regional install dimension on older Czech and Slovak drawings and in the catalogues of manufacturers who served that market. Nobody is issuing new parts to it for fresh designs.

How to read the size code

Unlike AS568 dash numbers or ISO 3601 letter codes, 02 9281 does not use a catalogue number. The size is simply the ring itself written as d1 × d2 — inner diameter × cross-section (cord thickness) in millimetres. So "8 × 2" means an inner diameter of 8 mm and a cord thickness of 2 mm; "100 × 5" means 100 mm inner diameter, 5 mm cord. A material or variant suffix is sometimes appended: for example .2 denotes NBR, while .9 denotes a special elastomer such as polyurethane or silicone. The two dimensions are the whole identity — there is no dash, no series letter and no catalogue index to decode.

The inner-diameter range runs from 6 mm to 500 mm. The cross-section values are round numbers — 1.8, 2, 3 and 5 mm are the confirmed thicknesses. Keep this thickness set in mind, because it is the detail that separates 02 9281 from the international standards.

Why a 02 9281 ring is not an ISO 3601 ring

It is tempting to assume that an old metric ring is just an ISO 3601 ring under a different name. For 02 9281 that assumption is wrong, and the cross-section gives it away. ISO 3601 (and the equivalent DIN 3771) is built on five standard cord thicknesses: 1.80, 2.65, 3.55, 5.30 and 7.00 mm. 02 9281 uses 2, 3 and 5 mm. Those numbers do not line up. A 3 mm cord is not a 2.65 mm cord and not a 3.55 mm cord; a 2 mm cord is not 1.80 mm.

The consequence is that even where an inner diameter happens to coincide between the two systems, the parts are genuinely different rings. A 02 9281 ring is not an ISO ring relabelled. The modern metric series that does derive from ISO 3601 / DIN 3771 is the sibling standard PN 02 9283 — not 02 9281. So if a drawing references 02 9281, do not silently substitute the nearest ISO 3601 catalogue size on cross-section alone; the cord thickness will be off.

Which O-ring to use today

There are two situations, and the right answer differs for each.

For a new design

Do not design to 02 9281. It is cancelled, regional and not stocked as a current series. Design instead to the modern ISO 3601 / DIN 3771 sizing — the series that PN 02 9283 follows. Those sizes are documented in the dimension tables on oringy.sk and are held in stock, so a new groove drawn to ISO 3601 can be supplied from a live catalogue rather than a discontinued one. Pick the inner diameter and the standard cord thickness from the ISO 3601 table, then take that size straight into the configurator to see the rings available in stock.

To replace an existing 02 9281 part

Do not order from the old code alone. Because the standard is cancelled and its full size table is not reliably available, the dependable method is to measure the real ring. Take a caliper to the part you are replacing, read its inner diameter and its cord thickness in millimetres, and match by dimension. If the ring is too worn or hardened to measure cleanly, our guide on measuring an unknown O-ring shows how to recover the original dimensions. Match on the measured d1 × d2 — that is what the seal cares about, regardless of which standard the drawing once cited.

Frequently asked questions

Is PN 02 9281 the same as ČSN 02 9281 and STN 02 9281?

Yes — they are the same standard expressed under the Czechoslovak (ČSN), later national, and Slovak (STN) prefixes. STN 02 9281 was issued in 1960 and the ČSN edition has been valid since 1961. The dimensions are identical; only the prefix differs by jurisdiction.

Is 02 9281 the same as ISO 3601 or DIN 3771?

No. ISO 3601 and DIN 3771 use cord thicknesses of 1.80, 2.65, 3.55, 5.30 and 7.00 mm, whereas 02 9281 uses 2, 3 and 5 mm. The cross-sections do not match, so a 02 9281 ring is not an ISO ring under another name. The metric series that does follow ISO 3601 is the sibling standard PN 02 9283.

What does the .2 or .9 code after the size mean?

The number after the size is a material or variant code, not part of the dimension. For example .2 indicates NBR and .9 indicates a special elastomer such as polyurethane or silicone. The sealing dimension is only the d1 × d2 in front of it.

Where can I get the full 02 9281 size table?

The complete enumerated table lives only in the original copyright-protected norm, which is cancelled and not freely reproducible, so we deliberately do not publish it. For practical sourcing you do not need it: measure the real ring in millimetres and match by dimension, or design to the current ISO 3601 series instead.

How to proceed

If you are starting a new design, open the ISO 3601 dimension tables on oringy.sk, choose the inner diameter and a standard cord thickness, and take that size into the configurator to find rings in stock. If you are replacing an existing 02 9281 part, measure its inner diameter and cord thickness with a caliper and match by those millimetres — and if the old ring is damaged beyond reading, follow our guide on how to measure an unknown O-ring. Confirm the material suits your medium against the chemical-compatibility table. And if you have a 02 9281 dimension that does not map cleanly onto a current size, write to Filip — legacy Czechoslovak sizes are usually something we can track down.