Cross Section: .331" (8.40mm) round solid Buna-N 70 duro O-Ring cord, sold by the foot
Part number N70.331 is .331 inch (8.40mm) round black Buna-N 70 duro nitrile O-Ring cord stock from The O-Ring Store LLC, sold by the foot and cut to whatever length you order. The cross section is held to Class E2 Precision, which is plus or minus .027 inch (0.70mm). This is a cross section outside the AS568 inch series, which is exactly the kind of size cord exists to cover. Service range is -40°F to +250°F (-40°C to +121°C). Weight is .05596 pounds per foot. Every reel is measured and certified on the Olypsys® O-Ring Measurement System before it ships.
Get the Seal You Need Without Waiting on a Mold
Cord stock is how you seal a size nobody tools. You cut it to length, splice or vulcanize the ends, and you have a ring in the diameter your machine actually needs instead of the closest catalog size. We spool 100 foot runs in house, so a short order ships the same day instead of sitting in a queue. Longer runs depend on factory production and we will tell you the real lead time up front. This is the material people reach for on large diameter flanges, manway covers, tank lids, one-off repairs, field splices, and any groove where the standard AS568 chart runs out of sizes.
Operating Temperature
-40°F to +250°F (-40°C to +121°C) in continuous service. Nitrile stiffens as it gets near the cold end and starts to take a compression set at the top end, so if your application lives at either edge for long stretches, call us and we will point you at a better compound.
Material and Performance Details
Buna-N, also called nitrile or NBR, is the workhorse elastomer for petroleum service and it is the most common sealing compound sold anywhere. It is a copolymer of butadiene and acrylonitrile, and the acrylonitrile content is what sets its personality. More acrylonitrile buys you oil and fuel resistance and costs you low temperature flexibility. Less acrylonitrile does the opposite. A 70 durometer nitrile like this one sits in the middle, which is why it fits so many jobs. It has excellent tensile strength, very good abrasion and tear resistance, and it holds a seal against pressure better than most soft compounds.
Buna-N is also the best behaved cord in the whole lineup from an extrusion standpoint. It is sulfur cured, it has low die swell, and it does not need a post cure, so it comes off the line dimensionally stable. That is why nitrile holds Class E2 comfortably. Silicone and peroxide cured EPDM both need a post cure and shrink more, so they cannot be held as tight. Nitrile cord that misses E2 points at a line problem, not a physics problem.
What it does not like is sunlight and ozone. Nitrile has poor weather resistance and will surface crack if you leave it outdoors in UV for a season. Store it in the dark, keep it out of the ozone from motors and welders, and it will hold up for years.
Excellent Compatibility
- Petroleum based hydraulic oils
- Gasoline, diesel, kerosene and hydrocarbon fuels
- Mineral and vegetable oils and greases
- Water and most water glycol fluids
- Silicone oils and greases
- Dilute acids and bases at room temperature
- Propane, butane and LPG
Limited or Conditional Compatibility
- Hot water and low pressure steam, better handled by EPDM
- Concentrated acids
- Alcohols and glycols at elevated temperature
- Outdoor and UV exposure, expect surface checking
Not Compatible
- Ketones such as MEK and acetone
- Aromatic hydrocarbons such as benzene and toluene
- Chlorinated hydrocarbons
- Phosphate ester hydraulic fluids such as Skydrol
- Strong oxidizing agents
- Ozone, direct sunlight and weathering
Size and Cross Section Tolerance
O-Ring cord is a single round cross section. There is no inside diameter or outside diameter on the cord itself, because the ID and OD only exist once you cut the cord and form it into a ring. The number that matters when you buy cord is the cross section, and here is what you get:
- Cross Section, inch: .331" nominal
- Cross Section, metric: 8.40mm nominal
- Tolerance: plus or minus .027" (0.70mm), ARPM Class E2 Precision
- Actual E2 window: 0.304" to 0.358" (7.70mm to 9.10mm)
- Part number: N70.331
- Weight: .05596 pounds per foot
Cross sectional tolerance on extruded cord is set by the ARPM Rubber Handbook, 7th Edition, Table 13, and by ISO 3302-1, Table 4. The numbers in both are identical. ARPM is the group most of the industry still calls RMA. There is no separate table for nitrile, Viton or anything else. The class is what gets negotiated, and the material only determines which class a mill can realistically hold.
| Class | Name | Tolerance on .331" | Low to high |
|---|
| E2 | Precision | ± .027" (0.70mm) | 0.304" to 0.358" |
| E3 | Commercial | ± .039" (1.00mm) | 0.292" to 0.370" |
Look at the E3 row, because it is the most useful line on this page. A .331 cord built to E3 Commercial can measure anywhere from 0.292 to 0.370 and still be a conforming, in spec product. That is the cheap cord on the market. We hold E2 instead, which cuts that window down to 0.304 to 0.358. Nothing is broken when you see how wide the commercial class runs. That is what the standard says, and anybody selling you cord who does not tell you that is doing you a disservice.
We hold Class E2 Precision on solid cord and E2 is what we guarantee. Most nitrile that comes through our door measures well inside that window. Underpromise on paper, overdeliver in the box.
Why Extruded Cord Cannot Hold Molded Tolerances
Five real reasons, in plain terms. Die swell, where rubber leaves the die larger than the die opening and rebounds the instant it exits. Cure shrinkage, where it shrinks back during vulcanization by an amount that varies with the polymer. No cavity, because a molded ring cures inside a steel mold that physically holds the dimension while the chemistry happens and extruded cord cures in open air with nothing holding its shape but its own stiffness. Post cure, which nitrile does not need but silicone, peroxide cured EPDM and FKM all do. And time on the reel, because cord relaxes, takes a set where it is coiled, and responds to temperature and humidity. Cord that measured dead nominal at the mill can measure differently six months later on a shelf and neither reading is wrong.
How to Measure Cord Correctly
Most dimensional arguments turn out to be measurement arguments. Four habits settle almost all of them. Use light pressure and let the caliper close on the cord instead of cranking it down, because hand pressure on a 70 duro compound reads .002" to .004" under actual. Measure in three places 90 degrees apart and average them, since cord goes slightly oval on the cure conveyor. Let it sit at room temperature first, because rubber off a cold truck is not the size it will be on the bench. And measure a straight piece, not a coiled one. An optical gauge removes the pressure variable entirely, which is exactly why we measure and certify on the Olypsys® O-Ring Measurement System instead of a hand caliper.
Figuring Out How Much Cord to Buy
To make a ring from cord, the length you need is the centerline circumference of the finished ring. The formula is ((OD + ID) / 2) x Pi = Length, where Pi is 3.14159. If you only know the OD, the inside diameter is ID = OD - (2 x C/S). Our free O-Ring Cord Cut-Length Calculator does that math for you. Order a little extra for the splice and for trimming. If the gland is tight and the cross section has to be exact, stop buying cord and let us quote you a custom vulcanized O-Ring in the same nitrile compound, which cures the joint in a mold and gives you real molded cross section control on a size no molder tools.
Applications and Use Cases
This is a heavy cross section built for pressure and for big diameters. It carries enough squeeze to seal a rough or worn groove that a lighter cord would leak past.
- Large diameter flange and manway gaskets on tanks and pressure vessels
- Heavy hydraulic cylinder heads and end caps
- Filter housings, strainer bodies and separator covers
- Mining, logging and heavy equipment repair in the field
- Pump casings and split housings on large industrial pumps
- Access hatches and bolted covers on process equipment
- Custom vulcanized rings in diameters no molder tools
Key Specifications
- Part number: N70.331
- Material: Nitrile, Buna-N, NBR
- Durometer: 70 Shore A
- Color: Black
- Profile: Round solid extruded cord
- Cross section: .331" (8.40mm)
- Tolerance: ARPM Class E2 Precision, ± .027" (0.70mm)
- Governing standard: ARPM Rubber Handbook 7th Ed. Table 13, ISO 3302-1 Table 4
- Temperature range: -40°F to +250°F (-40°C to +121°C)
- Specific gravity: 1.25
- Weight per foot: .05596 pounds
- Sold by: the foot, cut to your length
- Spooling: 100 foot runs spooled in house
- Cure system: Sulfur cured, no post cure required
Why Buy Your O-Ring Cord from The O-Ring Store LLC
We are an ISO 9001:2015 certified O-Ring and sealing distributor in Clarkston, Washington, and we have been doing this since 2007. We stock the cord, we spool it here, and we cut it to your length instead of making you buy a full reel. Every batch gets measured and certified on the Olypsys® O-Ring Measurement System, which is an optical system, so the numbers we give you are not somebody's caliper reading. If you need a tolerance we do not publish, or a size that is not on the list, or a vulcanized ring made from this same compound, call us and we will quote it. We make getting O-Rings easy.
Common Questions
What are the exact dimensions of this cord?
The nominal cross section is .331 inch, which is 8.40 millimeters. Held to ARPM Class E2 Precision at plus or minus .027 inch (0.70mm), a conforming piece measures between 0.304" and 0.358" (7.70mm to 9.10mm). Cord has no inside or outside diameter of its own. Those dimensions only exist after you cut it and form it into a ring.
What fluids is Buna-N 70 cord compatible with?
Petroleum based hydraulic oils, gasoline, diesel, kerosene, mineral and vegetable oils, water, silicone oils and greases, propane and LPG, and dilute acids and bases at room temperature. Stay away from ketones like MEK and acetone, aromatic hydrocarbons like benzene and toluene, chlorinated hydrocarbons, phosphate ester hydraulic fluids like Skydrol, and strong oxidizers. Also keep it out of direct sunlight and ozone, because nitrile has poor weather resistance.
Why is cord tolerance so much wider than a molded O-Ring?
Because there is no steel cavity holding the dimension while the rubber cures. Extruded cord swells coming out of the die, shrinks during cure, and keeps moving on the reel afterward. A molded ring cures inside a mold that physically holds it. The standard is written around that reality, and it gives cord a window six to nine times wider than a molded ring at the same cross section. If your gland needs molded accuracy, ask us for a vulcanized ring instead.
Is this an AS568 size?
No. .331 inch (8.40mm) is not one of the five AS568 inch cross sections, which are .070, .103, .139, .210 and .275. That is not a problem, it is the whole point of cord. Metric equipment and imported machinery use cross sections the AS568 chart never covered, and cutting cord is how you seal them without tooling a mold. If you need it as a finished ring, we can vulcanize it for you.
How much does it weigh?
.05596 pounds per foot. That figure is based on a 1.25 specific gravity for 70 duro nitrile and includes an allowance over the raw calculated weight, so it is the number to use for freight and packaging.
