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AF Hydro · ready stockO‑rings to JIS B2401 & AS568, 1.90 mm to 8.40 mm section
The seal that does the work in every four‑bolt flange and most fittings, in NBR, Viton FKM, EPDM and silicone. G, P and V series to JIS B2401, plus AS568 dash sizes. Full size tables with the cross‑section for every series, and the fluid and temperature limits that decide the compound — below.
What an O‑ring is, and why the number on the packet matters
One dimension short and it either will not fit the groove or it will not seal.
An O‑ring is a moulded elastomer torus that sits in a machined groove and is squeezed between two faces. The squeeze — not the metal contact — is what holds the pressure, and system pressure then pushes the ring harder against the low‑pressure side of the groove, which is why a correctly sized O‑ring seals better as the pressure rises rather than worse.
That only works if two dimensions are right: the inside diameter, which decides whether the ring sits in the groove without stretching or bunching, and the cross‑section — the thickness of the cord — which decides how much squeeze there is. An ID that is close enough will still fail if the section is wrong, and this is where most wrong‑part orders come from: the size code carries both, and people quote only the number.
The compound is the third decision, and it is not a detail. The same size in the wrong material swells, hardens or dissolves. EPDM in mineral oil is the classic write‑off — it swells until it will not go back in the groove. Fluid and temperature are on the enquiry list for a reason.
Specification
Everything that has to be on the enquiry, in one table.
| Property | Specification |
|---|---|
| Standards | JIS B2401 (G, P and V series) · AS568 dash sizes |
| Series | G — static · P — dynamic · V — vacuum |
| Cross‑sections | 1.90 · 2.40 · 3.10 · 3.50 · 4.00 · 5.70 · 6.00 · 8.40 mm — eight families, fixed per series |
| Compounds | NBR nitrile · FKM (Viton®) · EPDM · silicone VMQ |
| Hardness | Commonly 70 Shore A for general service; HS90 where the groove is a high‑pressure flange face |
| Sizes | Metric and imperial. Non‑standard and one‑off sizes on request |
| Sectors | Oil & gas, marine, engineering and general industry |
| What we need to quote | ID and cross‑section, or the size code; then the fluid and the temperature |
If you only have the old ring, send us the ring or a photo of it beside a rule. If you only have the groove, send the groove diameter and width. Either is enough — a size code on its own without the series letter is not.
Buying O‑rings in Malaysia and Singapore
Where the stock sits, what it costs to get it to you, and how fast it moves.
Stock, delivery and ordering
- Where the stock is — the office is in Kuala Lumpur, and seals are held in two warehouses: Johor Bahru and Singapore. Addresses and direct lines are on the contact page.
- Dispatch — stock items leave the same or next working day.
- Delivery within Malaysia — 3 to 5 working days after dispatch. On an urgent stocked item, message us on WhatsApp and say it is urgent.
- Delivery cost — already inside the product price, along with transport, currency conversion, duties and taxes. Nothing is added afterwards.
Non‑standard sizes. Anything outside the JIS and AS568 tables is made rather than picked, so the lead time comes back on the quote. Send the ID and the section and we will tell you whether it is a stock size before you design around it.
How an O‑ring enquiry runs
Four things and we can price it without going back and forth:
- Size — the code with its series letter (
G55,P22,V150), or the ID and cross‑section in millimetres. - Compound — NBR, Viton FKM, EPDM or silicone. If you are not sure, give us the next two instead.
- Fluid and temperature — what it seals against and how hot it gets. This is what actually picks the compound.
- Quantity — and whether it is a one‑off repair or a stocked spare you will reorder.
A product specialist prices it within two working hours. Replacing a seal on a square flange or a SAE flange? Give us the flange size and series instead — the ring is fixed by the flange, so we can look it up.
JIS B2401 or AS568?
Two size systems, and the machine you are working on decided which one years ago.
JIS B2401 — the metric system
The Japanese standard, and the one that turns up on Japanese and Korean‑built machinery, shipboard hydraulics and the JIS flange ranges we stock. Sizes are coded by a letter and a number — G55, P22, V150 — where the letter is the duty and the number is close to, but not the same as, the inside diameter in millimetres.
Every JIS square flange on this site takes a B2401 ring, keyed to the flange size and series rather than chosen by the fitter.
AS568 — the inch system
The American standard, coded as a dash number — −210, −219, −228. It is what SAE J518 flanges and most American and European hydraulic equipment use. The dash number fixes both the ID and the section, and in the −200 series the section is 3.53 mm across every size in it.
On an SAE flange the dash number is fixed per size and is the same for Code 61 and Code 62 — so the O‑ring is one of the few parts that does not change with the code.
Do not convert between them. There is no clean equivalence: a B2401 ring and an AS568 ring of similar diameter will differ in section, and a section that is 0.4 mm out changes the squeeze enough to leak or to extrude. Order in the system the groove was cut for.
G, P or V — what the letter means
It is the duty of the seal, not a size band. Getting it wrong fits a static ring to a moving shaft.
| Letter | Duty | Cross‑sections used | Where it belongs |
|---|---|---|---|
| G | Static | 3.10 · 5.70 mm | Faces that are clamped and do not move — flange joints, covers, housings. Every JIS square flange ring is a G. |
| P | Dynamic | 1.90 · 2.40 · 3.50 · 5.70 · 8.40 mm | Sliding or rotating duty — cylinder pistons, rods, spool bores. Also used static where the groove was cut for a P. |
| V | Vacuum | 4.00 · 6.00 mm | Vacuum flanges and low‑pressure gas joints, where the ring has to seal against being pulled inward rather than pushed out. |
The letter is part of the size, not a note on it. P150 and G150 both exist, both take a 5.70 mm section, and their inside diameters are 149.60 and 149.30 mm — 0.3 mm apart. An order that says “150” is not an order we can fill without asking, which is why we ask.
JIS B2401 sizes — the eight cross‑section families
The section is fixed by the family, not chosen. Find your family first, then the number.
A B2401 size code is a letter and a number, and the number does not equal the inside diameter — it is a nominal figure the standard assigns. What is fixed is the cross‑section: every code inside a family shares one section and one tolerance. Establish the family and you have half the specification.
| Section (mm) | Tolerance | Inches | Codes in the family | What it is for |
|---|---|---|---|---|
| 1.90 | ±0.08 | 0.075" | P3 – P10 | The smallest section held. Instrument and pilot lines. |
| 2.40 | ±0.09 | 0.094" | P10A – P22 | Small dynamic duty — spools, small bores. |
| 3.10 | ±0.10 | 0.122" | G25 – G145 | The square flange section. Every JIS square flange ring from 15A to 100A is in this family. |
| 3.50 | ±0.10 | 0.138" | P22A – P50 | Mid‑range dynamic. Note it overlaps P22 above on diameter but not on section. |
| 4.00 | ±0.10 | 0.157" | V15 – V175 | Vacuum, small to mid. |
| 5.70 | ±0.13 | 0.224" | P48A – P150 · G150 – G300 | Heavy duty. The G series jumps to this section at G150 — G145 is 3.10 mm, G150 is 5.70 mm. |
| 6.00 | ±0.15 | 0.236" | V225 – V430 | Large vacuum flanges. |
| 8.40 | ±0.15 | 0.331" | P150A upward | The heaviest section — large cylinder and press work. |
Two traps in this table. First, G145 to G150 is a section change, not a size step — 3.10 mm becomes 5.70 mm, so a groove cut for one will not take the other. Second, P22, P22A and P22.4 are three different rings: 21.80 mm ID at 2.40 mm section, 21.70 at 3.50, and 22.10 at 3.50. A tenth of a millimetre on the ID and more than a millimetre on the section.
The G series held for JIS square flanges
These are the rings that come in the box with a square flange pair. All 3.10 mm section, and the arithmetic is consistent: ID = the code number − 0.6, and OD = the code number + 5.6.
| Code | ID d (mm) | OD D (mm) | Section W (mm) | Flange size |
|---|---|---|---|---|
| G25 | 24.40 | 30.60 | 3.10 | ½" 15A |
| G30 | 29.40 | 35.60 | 3.10 | ¾" 20A |
| G35 | 34.40 | 40.60 | 3.10 | 1" 25A |
| G40 | 39.40 | 45.60 | 3.10 | 1¼" 32A |
| G50 | 49.40 | 55.60 | 3.10 | 1½" 40A |
| G60 | 59.40 | 65.60 | 3.10 | 2" 50A |
| G75 | 74.40 | 80.60 | 3.10 | 2½" 65A |
| G85 | 84.40 | 90.60 | 3.10 | 3" 80A |
The ring is keyed to the flange size and series together, not to the bore alone — a 50A flange takes a G60 in 210K but a different ring in 280K and 350K. If you are ordering a replacement seal rather than a whole flange, give us the flange size and series and we will look it up rather than have you measure a worn ring.
NBR, Viton FKM, EPDM or silicone?
The fluid picks the compound. Temperature only narrows it further.
| Compound | Typical range | Right for | Never for |
|---|---|---|---|
| NBR nitrile | −30 to +100 °C | Mineral hydraulic oil, diesel, grease, water at moderate temperature. The default, and the cheapest. | Phosphate ester (HFD) fluid, brake fluid, strong ozone or long UV exposure. |
| FKM (Viton®) | −20 to +200 °C | High temperature, fuels, solvents, most aggressive chemicals. What you move to when NBR hardens. | Hot water and steam, amines, low‑temperature duty — it goes stiff before NBR does. |
| EPDM | −40 to +130 °C | Water, steam, brake fluid, phosphate ester, weathering and ozone. Good outdoors. | Mineral oil of any kind. It swells until it will not return to the groove. This is the commonest write‑off. |
| Silicone VMQ | −60 to +200 °C | The widest temperature span, static joints, food and pharmaceutical duty, dry heat. | Dynamic duty and mineral oil — it tears and abrades. Static only. |
The figures above are typical continuous‑service ranges for the family, not a guarantee for a particular compound — a specific NBR can be better or worse than the band. Where the duty is near an edge, tell us the fluid, the peak temperature and whether it is continuous or intermittent, and we will confirm against the compound rather than the family.
Hardness matters as well as chemistry. General service is commonly 70 Shore A. High‑pressure flange faces use a harder ring — the JIS square flange ranges take NBR at HS90 — because a soft ring extrudes into the gap and gets nibbled. Fitting a 70 Shore ring where the drawing calls for HS90 will seal on the bench and weep in service.
How to identify an O‑ring you already have
Three measurements and a look at the groove. No code needed.
Measure the ring, not the packet
- Cross‑section first — put a vernier across the cord itself. This is the number that lands you in the right family, and it is the one people skip.
- Inside diameter — lay the ring flat, unstretched, and measure across the middle of the hole. Do not pull it taut.
- A used ring has taken a set — expect it to read slightly large on ID and slightly flat on section. Round to the nearest standard size rather than trusting the reading.
Read the groove and the failure
- Groove width and depth — if the ring is gone, the groove gives you the section directly, and the groove diameter gives you the ID.
- Hardened and cracked — it ran too hot for the compound. Move up, usually NBR to FKM.
- Swollen and soft — wrong compound for the fluid. Usually EPDM in oil.
- Nibbled at one edge — extrusion. The ring was too soft for the pressure, or the groove gap is too wide.
Send a photo with the ring beside a rule, or post us the old ring, to info@rfshydraulics.com or on WhatsApp. Identifying seals off a photo is routine work here — and the failure mode usually tells us more about what you actually need than the size does.
Fitting it
Four steps, and the damage almost always happens in the second one.
The four steps, in order
1. Clean and inspect the groove. Remove the old ring completely, and check the sealing faces for nicks, scoring and burrs. A scratch across the groove is a leak path that no amount of clamping force will close.
2. Do not drag it over a thread. Threads, sharp edges and keyways cut the ring on the way in, and the cut is on the inside where you will not see it. Tape the thread, use a fitting sleeve, or come at the groove from the other side.
3. Lubricate with the system fluid. A smear of the oil the system actually runs on. Not grease on a ring that will sit in a different fluid, and never a lubricant the compound is not rated for.
4. Seat it before you tighten. Make sure the ring is fully in the groove all the way round and not twisted, then bring the joint up evenly. A ring pinched at one point will hold on test and fail later.
Why the O‑ring joint earns its keep
The seal is a replaceable consumable in a machined groove, so a joint can be broken and remade many times over the life of the equipment. Nothing about the metalwork wears out — you renew the cheapest part and the joint is as good as new.
Because the pressure itself pushes the ring against the low‑pressure wall of the groove, an O‑ring joint seals harder as pressure rises. That is why the same ring covers a wide pressure band and why a joint that weeps at idle is usually a fitting fault rather than a pressure one.
Always change the ring when you open the joint. It has taken a set; refitting it starts from less squeeze than the design assumed.
Why buy O‑rings from RFS Hydraulics
Six things that decide whether a seal order goes smoothly.
Ready stock, not ordered in
Common JIS and AS568 sizes are held in Johor Bahru and Singapore rather than bought in against your enquiry, and stock items dispatch the same or next working day.
A named maker, not a mixed bin
The seals are AF Hydro, so a repeat order is the same compound from the same source rather than whatever the cheapest bag held that month. AF Hydro also supplies our copper pipe and coil.
Four compounds, not one
NBR, Viton FKM, EPDM and silicone, so the compound is a design decision rather than whatever the supplier happens to hold.
We check the fluid, not just the size
Give us the fluid and the temperature and we will confirm the compound. A right‑size ring in the wrong material is still a failed joint.
Identification from a photo
Send the old ring or a photo beside a rule. We identify seals off photographs every week, and the failure mode tells us as much as the dimensions.
The flange as well as the seal
We hold the square flanges, SAE flanges, bonded seals and fittings the ring goes into, so one order covers the joint.
Quoted in two working hours
A product specialist prices it within two working hours. Non‑standard sizes come back with the lead time on the quote. More about RFS Hydraulics.
Delivery within Malaysia is already inside the price. See the FAQ or the terms and conditions.
Frequently asked questions
Nine, from sizing and series letters through compounds to fitting.
What size O‑ring do I need?
Two dimensions, not one: the inside diameter and the cross‑section (the thickness of the cord). A size code carries both, but only if you include the series letter — G150 and P150 are different rings 0.3 mm apart on ID. If you have the old ring, measure the ID across the middle and the section with a vernier on the cord. If you only have the groove, give us the groove diameter and width.
What do G, P and V mean in a JIS B2401 size?
They are the duty. G is static — clamped faces that do not move, such as a flange joint. P is dynamic — sliding or rotating, such as a cylinder piston or a spool bore. V is vacuum. The letter also fixes which cross‑section family the ring belongs to, so it is part of the size rather than a note on it.
Can I use an AS568 O‑ring in a JIS groove?
No. There is no clean conversion between the two systems — a ring of similar diameter will have a different cross‑section, and a section that is a few tenths out changes the squeeze enough to leak or to extrude into the gap. Order in the system the groove was cut for: JIS B2401 for Japanese and Korean equipment and JIS flanges, AS568 for SAE J518 flanges and most American and European gear.
NBR or Viton — which one do I need?
NBR for mineral hydraulic oil, which is most jobs, and it is the cheaper ring. Viton FKM when the temperature is above what NBR takes, or the fluid is a fuel, a solvent or an aggressive chemical. Viton is not the better ring in general — it stiffens at low temperature sooner than NBR does, and it is wrong for hot water and steam. Tell us the fluid and the peak temperature and we will say which.
Why did my EPDM O‑ring swell up?
It saw mineral oil. EPDM is excellent in water, steam, brake fluid and phosphate ester, and it is destroyed by petroleum oil — it absorbs it, swells, and will not go back into the groove. It is the commonest wrong‑material failure we see. If the system runs on mineral hydraulic oil, the ring should be NBR or FKM.
What hardness should the O‑ring be?
70 Shore A is the general‑service default. A harder ring, HS90, is used where the groove is a high‑pressure flange face — the JIS square flange ranges specify it — because a soft ring extrudes into the joint gap under pressure and gets nibbled away at the edge. A 70 Shore ring fitted where HS90 was specified will seal on the bench and weep in service.
Which O‑ring does my flange take?
It is fixed by the flange, so you do not have to work it out. On a JIS square flange the ring is keyed to the size and the series together — a 50A takes a G60 in 210K and a different ring in 280K and 350K. On an SAE J518 flange the AS568 dash number is fixed per size and is the same for Code 61 and Code 62. Give us the flange size and series and we will look it up.
Do you supply non‑standard sizes?
Yes, on request. Anything outside the JIS and AS568 tables is made rather than picked off a shelf, so the lead time comes back with the quote rather than being a stock figure. Send the ID and the cross‑section and we will tell you whether it is already a stock size before you design around a special.
Can an O‑ring be reused?
Once it has been compressed in a groove it has taken a set, and refitting it means starting from less squeeze than the design assumed. On anything holding pressure, replace it — the ring is the cheapest part of the joint and the labour to open it again is not. Always inspect the groove at the same time: a scratch across the sealing face is a leak path a new ring will not close.
Catalogue
Direct PDF download — no form, no email address.
The download covers the flange‑keyed O‑rings: the SAE O‑ring and its seat, and the JIS square‑flange sizes for SHAB and SSAB 210 kgf/cm² and KAB 280 and 350 kgf/cm². It does not carry the general JIS B2401 list, which runs to several hundred codes across the eight cross‑section families — the families are tabulated on this page instead. For a code in neither, send it with the series letter and we will confirm the ID and section against the standard before you order.
O‑ring data for a specific flange range is published on the flange page itself — JIS square flanges and SAE flanges.
Send the ID, the cross‑section and the fluid.
Or the size code with its series letter, or a photo of the old ring beside a rule. A product specialist comes back within two working hours with the compound confirmed.