O-Ring Cord Tolerances (RMA / ARPM E1, E2, E3)

The short answer: extruded O-Ring cord is held to one governing table, and it is not material specific. Most cord on the market, ours included, is supplied to Class E2 (Precision). On the five common inch cross-sections that works out to ± .014" on .070 cord, ± .016" on .103 and .139, ± .020" on .210 and ± .027" on .275. Everything below explains where those numbers come from, why they are wider than a molded O-Ring, and what actually moves them.

Cutting cord to make a ring? The Cord Cut Length Calculator does that math. For dash numbers and molded sizes, use the Master O-Ring Size Chart.

Which standard sets cord tolerance, RMA or ARPM?

Both names point at the same document, and ARPM is the current one.

The governing table is the ARPM Rubber Handbook for Molded, Extruded, Lathe-Cut and Cellular Products, 7th Edition (2015), Table 13, Standard Cross-Sectional Tolerances.

ARPM is the Association for Rubber Products Manufacturers. It is the same organization most of the industry still calls RMA. The rubber products manufacturers split from the Rubber Manufacturers Association in 2010, formed ARPM and took the handbook with them. So “RMA tolerance table” and “ARPM tolerance table” mean the same thing. Both are correct.

The international equivalent is ISO 3302-1, Table 4, Tolerances on cross-sectional dimensions of unsupported extrusions. The numbers are identical to RMA/ARPM Table 13. Same three classes, same breakpoints, same values. That matters, because an overseas mill quoting to ISO 3302-1 E2 is quoting the exact same product as a domestic mill quoting RMA E2.

There is no fourth class, and there is no separate table for Buna, Viton®, silicone or EPDM. The class is what gets negotiated. The material determines which class is realistically achievable and what it costs.

The three classes: E1, E2 and E3

Cross-sectional tolerance, inch

Cross-sectional tolerance, millimeter

The notes published with the table

Three notes travel with the standard, and they are the most useful sentences in the whole document:

1. Tolerances on dimensions above 100 mm (3.94") are to be agreed between supplier and user.
2. Cross-sectional dimensions below 1 mm (.04") are impractical as a general rule.
3. Softer materials, and materials that require a post cure, need greater tolerances.

ISO 3302-1 adds a fourth worth quoting directly:

4. For certain synthetic rubbers, extrusion class E1 tolerances are not directly obtainable.

One correction worth knowing

Several published reprints show the inch column for the .25" to .39" band with E2 as ± .024". That is wrong. The metric master value is 0.70 mm, which converts to .0276", so the correct inch value is ± .027. If you are cross-checking numbers you found online and they do not match, work from the metric column and convert. The metric column is the master. The inch column is a soft conversion.

What does this mean for the cord sizes you actually buy?

The five inch cross-sections that match the AS568 series:

Molded cross-section tolerances in the last column are per AS568 / ISO 3601-1.

Common metric cord sizes:

Watch the band edges. A 4.0 mm cord sits at the top of the 2.5 to 4.0 band and gets ± 0.25 at E1. A 4.1 mm cord drops into the next band and gets ± 0.35. Nothing physical changed about the cord. That is simply how banded tolerance tables work, and it surprises people the first time they see it.

How does cord tolerance compare to a molded O-Ring?

A .070" cord built to E3 Commercial can measure anywhere from .050" to .090" and still be a conforming product. The molded .070 O-Ring sitting next to it on the shelf has to be between .067" and .073".

That is a window roughly six to nine times wider on cord than on a molded ring, and it holds across every size in the table. Nothing is broken when you see it. That is what the standard says.

Why can’t extruded cord hold molded tolerances?

Five real mechanisms, in plain terms.

Die swell. Rubber leaves the die larger than the die opening. It is elastic, it was under pressure and it rebounds the instant it exits. How much it swells depends on the compound, the filler loading, the extruder temperature and the line speed. Every one of those drifts a little across a shift.

Shrinkage during cure. After it swells, it shrinks back during vulcanization, and the amount varies by polymer. Silicone shrinks the most. Nitrile shrinks the least of the common four. The mill is chasing a moving target from both directions at once.

No cavity. A molded O-Ring cures inside a steel cavity that physically holds the dimension while the chemistry happens. Extruded cord cures in open air or on a conveyor with nothing holding its shape but its own stiffness. Soft cord sags and goes slightly oval before it sets. That is the whole ballgame, and it is why the standard is written the way it is.

Post cure. Anything that needs a post cure, which includes all silicone, all peroxide-cured EPDM and all FKM, goes through a second oven cycle and shrinks again. More cycles, more variation. The standard says this outright in note 3.

Time on the reel. Cord is not dimensionally frozen after it ships. It relaxes, it takes a set where it is coiled and it responds to temperature and humidity. Cord that measured dead nominal at the mill can measure differently six months later on a shelf, and neither measurement is wrong. That is why the honest practice in this industry is to buy cord to E1 and guarantee E2. It is not hedging on quality. It is telling the truth about a material that moves.

And then there is measurement. Rubber deforms under a caliper. Two people measuring the same piece of .139" cord with the same caliper get different numbers depending on how hard they squeeze. On a 70 duro compound, hand pressure reads .002" to .004" under actual. On soft or sponge material it is much worse. Before anyone rejects a reel, the first question is how it was measured, not what it measured.

Does the material change the tolerance?

The standard does not change by material. What changes is which class a mill can actually hold, and hold repeatably, run after run.

Buna-N / Nitrile (NBR)

The best behaved cord in the lineup. Sulfur cured, low die swell, no post cure required, dimensionally stable coming off the line. E1 is genuinely achievable from a good extruder and E2 is comfortable. Nitrile cord that misses E2 points at a line problem, not a physics problem.

EPDM

Middle of the pack. Peroxide-cured EPDM, which is what you want for steam and hot water service, requires a post cure and shrinks more than sulfur cured. That post cure is exactly what note 3 in the standard is warning about. Sulfur-cured EPDM holds dimension a little better but gives up compression set and steam performance. E1 is achievable from a good mill on a dedicated line. E2 is the honest published number.

Silicone (VMQ)

The hardest of the four to hold, and the one that generates the most questions. Three things stack up:

• Silicone shrinks the most of any common elastomer, roughly 2.5% to 4% depending on the compound and the filler loading.
• It is always post cured, which is the second half of note 3.
• It is soft and tacky in the uncured state, so it deforms under its own weight on the cure conveyor before it sets. Round cord goes slightly oval.

This is the material ISO 3302-1 has in mind when it says class E1 is not directly obtainable for certain synthetic rubbers. E2 is realistic and honest. E1 silicone cord exists, but it comes off a dedicated line with 100% inline inspection and it is priced accordingly.

One more note on silicone: measure it with light pressure or do not touch it at all. A caliper cranked down on 70 duro silicone reads .003" to .005" under actual every single time, and that alone accounts for a lot of “out of spec” phone calls.

Viton® / FKM

Worth knowing before you order: there is effectively no 70 durometer FKM cord in the market. The industry standard for FKM cord is 75 durometer. Published extruder catalogs run 55, 60, 65, 75, 76, 80 and 90. There is no 70 in the lineup. If a print calls out 70 duro Viton® cord, 75 is what the application wants and 75 is what the market builds, the same reason 75 duro is the standard for molded FKM O-Rings.

FKM cord itself extrudes and holds dimension better than any of the other three. The benchmark numbers in the next section prove it. Good FKM cord beats E1 without breathing hard.

Does durometer change the tolerance?

Yes, and it is the single most predictable variable on the page. Softer cord holds looser.

The numbers below are published tolerances from a precision fluoroelastomer extruder (Eagle Elastomer, Peninsula OH, extruding fluoroelastomer since 1983, post cured, 100% virgin polymer, 100% laser inspected, ISO 9001 and AS9100). They are the right benchmark for the top of the market.

The ceiling: what the best cord actually holds

75 duro black Viton® A cord against the standard:

So the ceiling is real. Precision FKM cord runs at roughly half the E1 allowance. Anyone who says extruded cord “can’t” hold tight dimensions is describing one line, not the process.

The durometer effect, in real numbers

Same manufacturer, same polymer family, same equipment. Only the durometer changes:

Softer cord holds looser. Every size, no exceptions, roughly 30% to 75% looser at 55 to 65 duro than at 75 duro. That is note 3 of the standard, demonstrated. If you need tight cord and the application allows it, go up in durometer.

The compound effect

Same manufacturer, same durometer class, different compound:

The specialty polymers (GF-S, GLT-S, F type) run looser than straight Viton® A, and the commodity brown runs looser than the black. Same building, same equipment, same inspection. The compound itself sets the floor. That is the honest answer to why one FKM cord costs more than another and why the two do not measure the same.

Sponge cord is a different animal

Sponge cord is not covered by Table 13 at all. It has its own tables and they are far wider, for a good reason: you are extruding a material that is chemically expanding while it cures.

Sponge is not graded in Shore A durometer

Sponge rubber is graded by compression deflection per ASTM D1056, in grades like 2A1, 2B2 and 2C3, not in Shore A points. A request for “70 durometer sponge cord” is asking for something that does not exist, and any supplier who quotes it is guessing. This is the single most useful thing to know before buying sponge. Neoprene/EPDM blend sponge cord, the common one, lands in the D1056 2C grade family: closed cell, oil resistant, medium firmness.

Table 37, BER, round, square and rectangular sponge

Metric equivalents: 3.2 to 12.5 mm ± 0.80 / ± 0.80 / ± 1.00; 12.5 to 25 mm ± 1.25 / ± 1.25 / ± 2.00; 25 to 50 mm ± 1.60 / ± 2.00 / ± 4.00; 50 to 80 mm ± 2.50 / ± 3.20 / ± 5.00.

Two things to watch on this table. Some printings show BER 2 for the .50 to 1.0" band as ± .500", which is a decimal typo; the metric master is 1.25 mm, or ± .049", so the correct value is ± .050. And Table 37 does not cover anything under .125" (3.2 mm). Below 1/8" there is no published class, and the tolerance is whatever you and the mill agree to in writing.

Table 36, BEC, irregular and cored sponge

For hollow and profile sponge:

Class 1 here is intended only for high volume automotive work, and the standard specifically says Class 1 should not be applied to soft grades below 63 kPa (9 psi) compression deflection. The softest sponge cannot be bought to BEC 1 at all.

Table 33, SEC-3, extruded closed cell silicone

Silicone sponge gets its own table, and the small sizes carry a plus-biased tolerance rather than a symmetrical one:

Sponge cut length, Table 41, SLD

Sponge in perspective

A .250" sponge cord at BER 3 is ± .040", meaning .210" to .290". The same .250" solid cord at E3 is ± .031", and at E1 it is ± .014". Sponge cord runs roughly three times looser than solid cord at the same nominal size, and that is the standard working as designed.

Sponge measurement is its own problem too. A caliper on soft closed-cell sponge compresses it 10% or more. Sponge has to be measured optically or with a very light force gauge, and every question about sponge dimensions should start with how it was measured.

What is the length tolerance on a reel of cord?

People forget this one and then argue about a 100 foot reel that measured 98 feet. Table 16, unspliced extrusions:

Over 160 inches it converts to a percentage. On a 100 foot reel at L2 that is ± 0.32%, or about ± 3.8 inches. At L3 it is ± 0.50%, about ± 6 inches. Both are in spec.

The standard also notes that very soft and high-tensile stocks need special consideration on length, and that rubber has to be conditioned at room temperature before length is measured because it stretches and compresses so easily. Cord pulled off a cold truck and measured immediately will read short.

How to measure cord correctly

Most dimensional disputes are measurement disputes. Four habits settle almost all of them:

1. Use light pressure. Let the caliper close on the cord, do not crank it. Hand pressure on 70 duro reads .002" to .004" under actual, and worse on soft or sponge material.
2. Measure in three places, 90 degrees apart. Cord goes slightly oval on the cure conveyor. One reading tells you nothing about the piece. Average the three.
3. Let it sit at room temperature first. Rubber off a cold truck or out of a hot warehouse is not the size it will be on the bench. The standard calls for conditioning.
4. Measure a straight piece, not a coiled one. Cord takes a set where it is wound. Pull a length off, let it relax, then measure.

An optical or non-contact 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.

When to stop using cord and order a vulcanized ring

Cord is the right answer for large diameters, one-off repairs, field splices, and any size that does not exist as a molded ring. It is the wrong answer when the gland is tight and the cross-section has to be exact.

If the design needs molded accuracy, the better move is a vulcanized ring made from the same compound, which cures the joint in a mold and lets you hold a real cross-section tolerance on a size no molder tools. Ask us and we will quote it. For groove math either way, use the Groove Charts and the Groove Calculator.

What The O-Ring Store publishes

We publish Class E2 (Precision) on solid cord and BER 3 on sponge, and we buy to Class E1 wherever the mill can hold it. Most material that comes through the door lands inside E1. E2 is what we guarantee.

Underpromise on paper, overdeliver in the box. Every batch we ship is measured on the Olypsys® O-Ring Measurement System from The O-Ring Store LLC, an ISO 9001:2015 certified distributor. Shop O-Ring cord stock or ask us for a tolerance we do not list.

A word about the factories that make this cord

Everything above describes classes and limits, and it is easy to read a tolerance table as a list of what a mill failed to do. It is not. The factories running extrusion lines are producing the closest tolerance they are able to hold, and they are doing it on a process that fights them the whole way.

Extrusion is not molding. There is no steel cavity holding the dimension while the chemistry happens. The compound swells coming out of the die, shrinks through the cure, moves again through a post cure, and every one of those steps responds to the batch of polymer that came in that week, the humidity in the building, the temperature of the barrel and the speed of the line. It varies batch by batch, and that is the nature of the process, not a lapse in care. Anyone who has run extruded cord or extruded rubber profiles knows how hard it is to hold a round cross-section to a few thousandths across thousands of feet of continuous material.

So when a reel measures at the wide end of its class, that is usually the honest output of a line that ran well. We expect tight tolerances, we buy tight tolerances and we inspect what arrives, but we also know what we are asking for. The mills we buy from do the best job they can on every run, and the tolerance classes in this standard exist precisely because the industry knows what this process can and cannot promise.

Sources: ARPM Rubber Handbook for Molded, Extruded, Lathe-Cut and Cellular Products, 7th Ed. (2015), Tables 13, 16, 33, 36, 37 and 41. ISO 3302-1, Table 4. AS568 / ISO 3601-1 for molded cross-section tolerances. ASTM D1056 for cellular rubber grading. Published extrusion tolerances from Eagle Elastomer, Peninsula OH. Values cross-checked against the metric master column, which governs where an inch conversion disagrees.