What to Send for a Quote

If you know the ring, this is enough to start:

Inside diameter and cross-section in mm or inch, or the size number from your parts list, such as an AS568 dash number.
The fluid and its maximum temperature.
Where it seals: a fixed joint, or a moving rod, piston or spool.
Material and hardness, if known, such as NBR 70.
The quantity, or the list of positions for a machine set, and the date you need them.

What an O-Ring Does, and Why It Leaks

An O-ring is a ring of rubber with a round cross-section, fitted in a groove. Squeezed between the groove and the part opposite, it seals; the system pressure then pushes it against the side of the groove and makes the seal tighter.

Leaks often come from a ring of the wrong material for the fluid or the heat, the wrong size for its groove, or damage when it was fitted, such as a cut from a thread or a sharp edge. The old ring often shows which: hard and flat from heat or age, swollen and soft from the wrong fluid, or nibbled at one edge where it was pushed into a gap.

Isolate the machine and release the pressure before you inspect or measure a seal. Before you open the line, lower or support any load it holds, let the oil cool and drain it, following the machine maker’s and the plant’s instructions.

We are a supplier, not the maker.

Where the Ring Seals

Where the ring sits decides its material, hardness and size. The drawings are schematic, not to scale.

Static Face Seals
The ring sits in a groove in one face and is squeezed by a flat cover or flange: filter housings, covers, pump and valve body joints, split flange heads. Nothing slides, so the widest range of materials can be used. With the pressure inside, the ring should rest against the outer wall of its groove.
Tell us: the groove’s diameters, width and depth if the old ring is lost, and whether the pressure is inside or outside.
Rod and Piston Seals
The ring sits in a groove on a piston or spool, or in the bore around a rod, and the parts slide past it. A moving seal needs a tough, wear-resistant material and a smooth, lubricated surface; hydraulic cylinders often use purpose-made rod and piston seals rather than plain O-rings.
Tell us: the rod or bore diameter, the groove size, and how far and how often the part moves.
Port and Fitting O-Rings
Straight-thread ports and O-ring face seal (ORFS) fittings seal on an O-ring, not on the thread: the ring is pressed into the chamfer at the mouth of the port, or held in a groove in the face of the fitting. The connection’s standard and size set the ring; the thread size alone does not identify it.
Tell us: the thread size and form, such as UNF, metric or BSPP, the port type from your drawing or parts list, and the type of fitting.
Backup Rings
Rings of PTFE or hard plastic fitted beside an O-ring, on the side away from the pressure, so that high pressure or a large gap cannot squeeze the O-ring into the gap between the parts. With pressure from both sides, a backup ring is usually fitted on each side.
Tell us: the groove width, and whether the old seal had one or two backup rings, solid, split or spiral.

O-Ring Materials

Choose the material for the fluid first, then check the temperature. For phosphate ester fire-resistant fluids, the material is checked against the exact fluid.

NBR, FKM, EPDM and Other O-Ring Materials

General guides, not ratings: check the compound. The figures are typical ranges for common grades of each material family, not limits of the rings we supply; the exact compound and the duty set the limits, and special grades reach further.

NBR, FKM, EPDM and other O-ring materials
Material and a typical range for common grades	Often used for	Common limitations: check the compound
NBR (nitrile): about -30 to +100 °C	Mineral hydraulic and lube oils, diesel, grease, air, and water at moderate temperatures	Glycol brake fluid, phosphate ester fluids, ozone and weather outdoors
HNBR: about -30 to +150 °C	Oils and fuels where NBR runs too hot or wears too fast	Phosphate ester fluids
FKM (fluorocarbon rubber): about -20 to +200 °C	Hot oils, fuels and many chemicals	Hot water and steam in common grades, glycol brake fluid, very low temperatures
EPDM: about -45 to +130 °C; peroxide-cured grades to about +150 °C	Water, steam, glycol coolants and glycol brake fluid	Mineral oils, fuels and grease, which make it swell
Silicone: about -60 to +200 °C	Static seals in air and over wide temperature ranges; food and drink equipment, in compounds approved for food contact	Moving parts, as it tears and wears easily; most fuels
PU (polyurethane): up to about +80 °C in common grades; some compounds go higher	Moving hydraulic parts that see wear and high pressure	Hot water and steam, which break down many grades; resistance varies by compound

O-Ring Sizes

An O-ring is named by its inside diameter and its cross-section, the thickness of the ring; two rings with the same inside diameter can have different cross-sections. Inch rings are usually ordered by an AS568 dash number, which fixes both. Metric rings are ordered by inside diameter x cross-section in mm, such as 20 x 2.5.

Metric series include sizes close to the inch ones, such as a 3.55 mm cross-section beside 3.53 mm. Their nominal sizes and tolerances differ, so a near match is neither automatically interchangeable nor automatically unusable: whether it seals depends on the groove. Give the series and size your drawing or parts list uses.

O-Ring Size Chart: Inch Series Cross-Sections

In each group of the AS568 inch series the cross-section stays the same, and the dash number rises with the inside diameter.

O-ring size chart: inch series cross-sections
Dash numbers	Cross-section (inch)	Cross-section (mm)
-004 to -050	0.070	1.78
-102 to -178	0.103	2.62
-201 to -284	0.139	3.53
-309 to -395	0.210	5.33
-425 to -475	0.275	6.99
-901 to -932	Varies with the port size	Varies

The -900 group is for straight-thread ports in inch fittings. Send the dash number and we confirm the inside diameter with the quotation.

How to Measure an O-Ring

Lay the ring flat and measure it without stretching it: the inside diameter across the hole, and the cross-section with a calliper closed only lightly on the ring.

For a large ring, measure round its outside with a tape and divide by 3.14 to get the outside diameter; the inside diameter is the outside diameter less twice the cross-section.

An old ring may have shrunk, swollen or taken a flat set. Where you can, measure the groove as well, its diameters, width and depth, and send them with the ring’s figures.

Drawing: an O-ring with its inside diameter marked, and a section through the ring with its cross-section marked (schematic, not to scale)

Before We Select an O-Ring

These details let us choose the ring. What our quotation will confirm: the item we propose and its specification, what we matched it to, the quantity and availability.

Before we select an O-ring
Detail	Why it matters
The fluid	The ring must not swell, shrink or harden in it: mineral oil swells EPDM, and glycol brake fluid attacks NBR.
Maximum and minimum temperatures	Heat hardens rubber and cold stiffens it, so a ring that seals when warm may leak on a cold start.
Pressure, and the gap between the parts	High pressure or a large gap needs a harder ring or a backup ring.
Static or moving	A moving seal needs a tougher material and a groove made for it.
Size: inside diameter and cross-section, dash number, or groove sizes	Measured as described above; a near size is checked against the groove before it is used.
Quantity, or the positions for a machine set, and the date	Lets us confirm availability and lead time with the quotation.

Example

From Our Quotations

We have quoted O-rings with split flanges for a pipeline job, and O-rings and seal rings as oil centrifuge spares.

Questions Buyers Ask

Can you supply a set of O-rings for a whole machine?

Tell us the machine, or send the list of positions and sizes, and we quote a set. A kit of the sizes a machine uses saves hunting for single rings during a breakdown.

When should I use an FKM O-ring?

FKM is the family name for fluorocarbon rubber, which is sold under several trade names. It is the usual choice where NBR would run too hot, or meets fuels and chemicals it cannot take; the materials table gives a typical range for its common grades.

What hardness should an O-ring be?

70 Shore A is the common hardness for general use. A 90 Shore A ring resists being squeezed into a gap at higher pressures, but needs more force to fit and seals less well at low pressure; a backup ring is the other way to stop that squeezing.

Does your invoice show the HSN code?

Yes. Our GST invoice shows the HSN or SAC code for each line.

Do you keep stock?

Availability is confirmed with each quotation.

What is the price?

Prices depend on the material, the size and the quantity, so we quote for each enquiry.