NOK O-Ring Materials — JIS Classes, Compound Codes and Test Data

Japanese O-rings are specified twice over: once by the JIS material class printed on the drawing, and once by the manufacturer's own compound code stamped into the production record. Understanding how NOK's material codes (A305, E116, S503, F201, G636, T222 and the rest) map onto JIS B 2401 classes — and what the underlying test data actually means — is the difference between an equivalent seal and a warranty claim. This page is the working reference we use when interpreting a Japanese O-ring specification.

Why a JIS O-ring carries two material identities

JIS B 2401-1 defines the classes: NBR-70-1 (mineral oil resistance), NBR-90, NBR-70-2 (gasoline resistance), FKM-70, FKM-90, EPDM-70, HNBR-70, HNBR-90, ACM-70, and the older Class 4C (silicone, per the 2005 edition). A class is a specification envelope: it fixes hardness, minimum tensile strength, minimum elongation, ageing limits, compression set limits and immersion limits. It does not fix a recipe.

A manufacturer then formulates a compound that sits inside that envelope and gives it an internal code. NOK's codes are letter-plus-number: A for nitrile, E for EPDM, S for silicone and fluorosilicone, F for fluoroelastomer, G for hydrogenated nitrile, T for polyacrylate, M for chloroprene, B for butyl, R for SBR and U for polyurethane. So A305 is the general-purpose 70 Shore A nitrile that satisfies JIS NBR-70-1 and old JASO class 1A; F201 is the general-purpose FKM 70 satisfying JIS FKM-70 and JASO class 4D.

NOK compound codes, hardness and service temperature

Temperatures below are guide continuous-service ranges; figures in brackets are short-excursion maxima. Read the low-temperature figure as the practical limit for a static seal and add margin for dynamic duty.

Nitrile (NBR) — the default family

Ethylene propylene (EPDM) — water, steam, coolant, brake fluid

Silicone (VMQ) and fluorosilicone (FVMQ)

Fluoroelastomer (FKM)

Hydrogenated nitrile (HNBR), polyacrylate (ACM) and the remainder

Reading the JIS test data

A JIS material class is defined by a test schedule, and NOK publishes both the standard (pass/fail) value and a representative measured value for each compound. The gap between the two is where engineering judgement lives: a compound that only just clears the standard will age very differently from one with substantial margin.

Note what the silicone table shows an engineer should never lose sight of: Class 4C only demands 3.4 MPa tensile strength and 60 % elongation. Silicone is specified for its temperature range and inertness, not its mechanical strength. It is a static-sealing material in almost every hydraulic context.

Compression set is the number that predicts life

Of everything on a datasheet, compression set at the service temperature is the best single predictor of how long a seal will hold. It measures the fraction of imposed deformation that is never recovered. An O-ring seals because it behaves as an energised spring; once set consumes the squeeze, the contact stress falls below system pressure and the joint weeps. As a working rule, keep measured set under 25 % for static duty and under 20 % for dynamic duty at the real service temperature — not at the convenient test temperature.

TR10 and the low-temperature limit

TR10 is the temperature at which a compound, held at 50 % elongation and cooled, recovers only 10 % of that deformation on rewarming. It is the closest laboratory analogue to "the temperature at which the ring stops behaving like a spring". Treat TR10 as the low-temperature limit of the seal and design with margin: an A305 nitrile with TR10 of −24 °C should not be trusted to seal a cold-start cylinder at −25 °C, even though the catalogue range says −24 °C.

Volume change: swell is tolerable, shrinkage is not

The JIS immersion limits are asymmetric for good reason — typically −8 % to +5 % in reference oil No. 1 and 0 % to +20 % in the more aggressive oil No. 3. Modest swell increases contact stress and can improve sealing; it only becomes a problem when the groove fill exceeds design (nominally 85 %, and never above about 95 % allowing for thermal expansion). Shrinkage means plasticiser is being extracted: the ring is simultaneously losing squeeze and getting harder, which is the classic precursor to a slow static leak. Any negative volume change beyond about −3 % should be investigated.

Series, standards and what they imply for material choice

JIS O-rings are organised into application series before they are organised into sizes, and the series carries an implicit duty:

• P series — dynamic and static, the main hydraulic and pneumatic series. Assume motion unless proven otherwise; specify with compression set and extrusion resistance in mind.
• G series — static (fixed) duty, wider cross-section relative to bore. Conformability matters more than extrusion resistance.
• V series — vacuum flange, to JIS B 2290. Low outgassing and low permeability dominate; this is where IIR and specific FKM grades earn their place.
• S / SS series — NOK's small-diameter, thin-section series for compact hardware, static and low-pressure. Section is small, so squeeze is small and set tolerance is tight.
• AS568 / ISO 3601 — the inch and international general-purpose series that a Japanese machine may still use for imported subassemblies.

Groove geometry follows JIS B 2401-2 (JIS B 2290 for vacuum flanges), and back-up rings follow JIS B 2401-4 in spiral (T1), bias-cut (T2) and endless (T3) forms. A hardness change is often the cheaper fix before reaching for a back-up ring: at 70 Shore A extrusion resistance is moderate, at 80 it is good, at 90 it is high but conformability is poor and installation damage becomes the new risk.

Section shapes and when an O-ring is not the answer

Material availability narrows sharply outside the O-ring profile. If a service condition forces an unusual compound, the O-ring section is usually the only one you will get it in without tooling.

Special-duty compounds worth knowing about

• Chemical and solvent resistance — dedicated grades (NOK's FP64 being the reference example) extend fluoroelastomer chemistry into media that attack a standard FKM: amines, ketones and strong solvents. Below these sit true perfluoroelastomers where cost allows.
• Hydrogen service — rapid gas decompression is the failure mode, not chemical attack. Special VMQ and purpose-formulated EPDM grades (E9079 class) are used because permeation and blistering behaviour, not oil resistance, govern.
• Hot water and steam — the WEX-series FKM grades (FP29, FP739) exist because standard FKM performs badly in hot water. Where the fluid is only water, EPDM remains the cheaper and better answer.
• Regulated service — compounds meeting the Food Sanitation Act (Ordinance No. 85) and the Water Supply Act (JIS S3200-7) are named formulations requiring dedicated production. Order by the approved compound reference and keep the certificate with the job.
• Colour coding — NOK does use colour on some grades (E575 violet, S503 dark blue, S740/S811 red, S924 green FVMQ, FP739 and T222 brown, U801 milky white). Treat colour as a useful cross-check, never as a specification.

Specifying a replacement

When replacing a Japanese O-ring, work in this order:

• Series and size designation (for example P22, G45, S8, or the AS568 dash number).
• Compound code if it is documented; otherwise the JIS class.
• Fluid, including additive package — a "mineral oil" that is a modern low-SAPS engine oil is a different chemical environment from reference oil No. 1.
• Continuous and peak temperature, plus the coldest start temperature.
• Pressure, pressure cycling and diametral clearance (extrusion risk).
• Motion type: static, reciprocating, rotary or oscillating.
• Any approval requirement — potable water, food contact, gas.

With those seven items the compound choice is usually determined. Without the fluid and the cold-start temperature it never is.