Quick answer: ISO 3601-1 defines two dimensional tolerance series for metric O-rings: series G (general purpose) and the tighter series A (aerospace and similar applications). The N, S, and CS letters that often follow an ISO 3601 size designation are not dimensional tolerance grades — they are the surface-quality acceptance grades of ISO 3601-3 (N general purpose, S special, CS critical service). AS568 took the opposite historical path: the original 1971 issue carried two tolerance classes (Class I and Class II) tied to compound shrinkage, but AS568A (1974) merged them into the single tolerance table that AS568B and later revisions retain. For a 3.53 mm CS O-ring, the ISO 3601-1 series G cross-section tolerance is ±0.10 mm; the equivalent AS568 -2XX series tolerance is ±0.004" (±0.10 mm). The groove machining tolerance (typically ±0.025–0.05 mm) usually contributes more to compression variation than the O-ring CS tolerance — run a worst-case tolerance stack before paying for tighter-than-standard dimensions.
O-ring tolerances define the allowable dimensional variation in inside diameter (ID) and cross-section (CS) from nominal. These tolerances directly affect sealing performance because both too much and too little compression cause failures: under-compression (groove too deep or O-ring ID too small relative to nominal) allows leakage; over-compression (groove too shallow or O-ring CS too large) increases friction, accelerates compression set, and risks extrusion in high-pressure applications. The two major standards — ISO 3601-1 (metric, with a general-purpose series G table and a tighter aerospace series A table) and AS568 (inch, a single tolerance table covering every dash number) — provide the dimensional foundation for O-ring groove design and procurement specification. This guide explains both tolerance systems, how to use them in tolerance stack analysis, which grade to specify by application, and the cost implications of each choice.
Why O-Ring Tolerances Matter
O-ring sealing depends on a predictable compression percentage — the fraction of the cross-section diameter that is compressed between the groove bottom and the mating surface:
Compression % = (CS − Groove Depth) / CS × 100%
If the O-ring CS is at its maximum tolerance and the groove depth is at its minimum tolerance simultaneously (worst-case over-compression), the actual compression can exceed design intent by 2–5 percentage points. If the CS is at its minimum and the groove is at its maximum (worst-case under-compression), the seal may have inadequate contact stress at the sealing face.
| Condition | Consequence |
|---|---|
| Under-compression (< 8–10%) | Insufficient contact stress → leakage, especially at low pressure |
| Correct compression (12–25% depending on application) | Designed sealing force → leak-free operation |
| Over-compression (> 30–35%) | Excessive friction (dynamic seals), accelerated compression set, extrusion risk |
| Extreme over-compression (> 40–50%) | Groove overfill, extrusion into clearance gaps, installation damage |
Key insight: The groove designer must specify groove depth with knowledge of the O-ring tolerance series. A groove designed for a nominal 3.53 mm CS O-ring to ISO 3601-1 series G (±0.10 mm CS tolerance) must account for O-rings as large as 3.63 mm in the worst-case stack — approximately 3% more CS than nominal. If the same groove is then supplied with O-rings molded to a negotiated precision tolerance of ±0.05 mm, the tolerance stack is narrower and the compression is more consistent.
ISO 3601-1 Dimensional Tolerances: Series G and Series A
ISO 3601-1 publishes two dimensional tolerance tables for metric O-rings:
- Series G (general purpose): The default for industrial O-rings; most commercially available metric O-rings are manufactured to this series. (The 2008 edition of the standard refers to the same tolerance level as Class B.)
- Series A (aerospace/precision): Smaller tolerance limits for aerospace and similar applications (Class A in the 2008 edition); its size table mirrors the AS568 inch series.
The letters that follow the size code in a complete ISO 3601 designation — N, S, or CS — are not dimensional tolerance grades. They are the surface-quality acceptance grades defined by ISO 3601-3, covered in the surface quality section below.
Inside Diameter Tolerance (±mm)
ISO 3601-1 does not use fixed ID tolerance steps; the tolerance is calculated from the nominal inside diameter d1:
- Series G: Tolerance d1 = ±[(d1^0.95 × 0.009) + 0.11]
- Series A: Tolerance d1 = ±[(d1^0.96 × 0.007) + 0.09]
Representative values below are evaluated at the upper limit of each ID range and rounded to two decimals using the standard's rounding rule:
| ID Range (mm) | Series G | Series A |
|---|---|---|
| ≤ 6.30 | ±0.16 | ±0.13 |
| > 6.30 – 10.0 | ±0.19 | ±0.15 |
| > 10.0 – 16.0 | ±0.24 | ±0.19 |
| > 16.0 – 25.0 | ±0.30 | ±0.24 |
| > 25.0 – 40.0 | ±0.41 | ±0.33 |
| > 40.0 – 63.0 | ±0.57 | ±0.46 |
| > 63.0 – 100 | ±0.82 | ±0.67 |
| > 100 – 160 | ±1.23 | ±1.00 |
| > 160 – 250 | ±1.82 | ±1.49 |
| > 250 – 400 | ±2.78 | ±2.29 |
| > 400 – 630 | ±4.22 | ±3.50 |
| > 630 – 1000 | ±6.48 | ±5.40 |
Cross-Section Tolerance (Series G, ±mm)
| Nominal CS d2 (mm) | Tolerance |
|---|---|
| ≤ 1.80 | ±0.08 |
| > 1.80 – 2.65 | ±0.09 |
| > 2.65 – 3.55 | ±0.10 |
| > 3.55 – 5.30 | ±0.13 |
| > 5.30 – 7.00 | ±0.15 |
| > 7.00 – 8.00 | ±0.18 |
| > 8.00 – 10.00 | ±0.21 |
| > 10.00 – 12.00 | ±0.25 |
Series A cross-section tolerances are tighter than the series G values above and are listed per size in Table 3 of the standard; for the common inch-derived cross-sections they correspond closely to the AS568 tolerances shown in the next section.
Practical significance for a 3.53 mm CS O-ring (series G):
- CS can range from 3.43 mm to 3.63 mm (0.20 mm total band)
- A groove designed for nominal compression will see compression variation of ±2.8% from O-ring CS variation alone — before accounting for groove dimensional variation
AS568 Tolerances: One Table Since AS568A
The current AS568 standard for inch-size O-rings defines a single set of tolerances that applies to every dash number. This was not always the case: the original 1971 issue of AS568 carried two tolerance classes — Class I and Class II — with the applicable class determined by the shrinkage rate of the rubber compound. When the standard was revised as AS568A in 1974, the two classes were merged into the single tolerance table that AS568B (2001) and later revisions retain. As the AS568B foreword notes, that single table conforms to the former Class I for O-rings with ID up to and including 0.50" (12.7 mm) and to the former Class II for larger IDs. Catalog references to "Class 1 / Class 2" or "precision / commercial" AS568 tolerance grades trace back to this withdrawn two-class system, not to the current standard.
AS568 Tolerance Values by Dash Number Series
| Dash Range | Nominal CS | CS Tolerance | ID Tolerance (scales with ID) |
|---|---|---|---|
| -004 to -050 | 0.070" (1.78 mm) | ±0.003" (±0.08 mm) | ±0.005" up to ±0.037" |
| -102 to -178 | 0.103" (2.62 mm) | ±0.003" (±0.08 mm) | ±0.005" up to ±0.055" |
| -210 to -284 | 0.139" (3.53 mm) | ±0.004" (±0.10 mm) | ±0.010" up to ±0.085" |
| -309 to -395 | 0.210" (5.33 mm) | ±0.005" (±0.13 mm) | ±0.005" up to ±0.120" |
| -425 to -475 | 0.275" (6.99 mm) | ±0.006" (±0.15 mm) | ±0.033" up to ±0.120" |
Unlike the CS tolerance, which is fixed per series, the AS568 ID tolerance increases with diameter — for example ±0.005" at -110 (0.362" ID), ±0.010" at -214 (0.984" ID), and ±0.033" at -425 (4.475" ID). The three smallest dash numbers (-001 to -003, with CS of 0.040–0.060") also carry a ±0.003" CS tolerance. The 900-series dash numbers cover straight-thread tube fitting boss gaskets with their own smaller cross-sections and are not part of the five CS series above.
Because the standard defines only one tolerance table, any AS568 O-ring sold to the standard meets these values. Tighter-than-standard dimensions are a negotiated manufacturer specification, not an AS568 class.
Tolerance Stack Analysis for Groove Design
A complete tolerance stack analysis for O-ring groove design must consider three sources of dimensional variation:
- O-ring CS tolerance (from the standard tolerance series selected)
- Groove depth tolerance (from the machining process and drawing specification)
- Mating surface dimensional tolerance (bore or rod dimension variation)
Worst-case compression analysis:
For a hydraulic cylinder rod seal:
- Nominal CS = 3.53 mm; ISO 3601-1 series G tolerance = ±0.10 mm → CS range: 3.43–3.63 mm
- Nominal groove depth = 3.04 mm (target 13.8% squeeze at nominal); machining tolerance ±0.05 mm → depth range: 2.99–3.09 mm
- Worst-case minimum compression: (3.43 − 3.09) / 3.43 × 100% = 9.9% (undercompression risk — marginally acceptable for hydraulic service target of 12–18%)
- Worst-case maximum compression: (3.63 − 2.99) / 3.63 × 100% = 17.6% (acceptable for hydraulic service)
With a negotiated precision CS tolerance of ±0.05 mm (half the standard series G band):
- CS range: 3.48–3.58 mm
- Worst-case minimum compression: (3.48 − 3.09) / 3.48 × 100% = 11.2% (improved — clear of the undercompression risk zone)
- Worst-case maximum compression: (3.58 − 2.99) / 3.58 × 100% = 16.5% (also reduced)
Halving the CS tolerance both lifts the minimum compression out of the risk zone and trims the maximum. For this application, a precision-molding agreement provides measurable improvement in the compression guarantee — likely worth the modest cost premium for a hydraulic cylinder. Note that ISO 3601-1 series A mainly tightens the ID tolerance (valuable for stretch control at large diameters); for a CS-critical stack, the practical levers are a negotiated CS tolerance and tighter groove machining.
Rule of thumb: The groove depth tolerance is typically the dominant contributor to compression variation. Even with O-rings molded to half the standard CS tolerance, a groove depth tolerance of ±0.10 mm produces larger compression variation than the O-ring CS tolerance. Investing in tighter groove machining tolerance (±0.025–0.05 mm) often has more effect on compression consistency than tightening the O-ring specification.
Tolerance Specification by Application
| Application | Recommended O-Ring Specification | Groove Depth Tolerance Target | Rationale |
|---|---|---|---|
| General industrial hydraulics (< 100 bar) | ISO 3601-1 series G / AS568 standard tolerance | ±0.05 mm | Standard tolerance adequate; groove tolerance is primary variable |
| Precision hydraulics, OEM automotive | Series G with ISO 3601-3 grade S surface quality, or negotiated precision CS tolerance | ±0.025–0.05 mm | Higher consistency for automated assembly and tighter compression range |
| High-pressure hydraulics (> 150 bar) | Series A dimensions or negotiated precision CS tolerance | ±0.025 mm | Tighter dimensional control improves extrusion resistance predictability |
| Vacuum seals (< 10⁻³ Torr) | Series A + ISO 3601-3 grade S or CS surface | ±0.025 mm | Consistent compression and a defect-free surface are critical for helium-leak-tight performance |
| Aerospace (SAE AS 1933, AMS-R-83485) | Series A + grade S/CS per the qualification document | ±0.013 mm (aerospace tolerance) | Safety-critical; documented compliance required |
| Medical devices and implantables | Series A + ISO 3601-3 grade CS | ±0.013–0.025 mm | Dimensional and surface control for qualification validation |
| Pharmaceutical process equipment | Series G + grade S surface quality | ±0.025–0.05 mm | Balance of quality and cost for GMP compliance |
| Food and beverage equipment | Series G / AS568, grade N surface | ±0.05 mm | Standard quality; color inspection more important than tight tolerance |
| Large static seals (ID > 300 mm) | Series G | ±0.10 mm | The ID formula tolerance widens with diameter; groove tolerance dominates the stack |
| Custom non-standard sizes | Tolerance set by mold capability (by agreement) | Match to application | Mold tolerance determines actual range |
Manufacturing Methods and Achievable Tolerance
The manufacturing method determines which tolerance grades are achievable in production:
| Manufacturing Method | CS Tolerance Achievable | ID Tolerance Achievable | Best Fit |
|---|---|---|---|
| Compression molding (standard) | ±0.08–0.13 mm | ±0.25–0.45 mm | ISO 3601-1 series G, AS568 standard |
| Transfer molding | ±0.06–0.10 mm | ±0.15–0.30 mm | Series G with margin; series A feasible |
| Injection molding | ±0.04–0.08 mm | ±0.10–0.20 mm | Series A; negotiated precision CS tolerance for small sizes |
| CNC lathe-cut (large diameters) | ±0.10–0.20 mm | ±0.50–1.00 mm | Series G only; dimensional control limited |
| Cord splice + bonding | ±0.10–0.15 mm on ID | ±0.50–2.00 mm | Series G; ID highly variable by splice method |
Why injection molding achieves tighter tolerances: Injection-molded O-rings cure in a closed mold with precise cavity dimensions and consistent fill pressure. The shorter cycle time also reduces post-cure dimensional variation. Transfer molding improves over compression molding by using a closed mold cavity but requires manual compound preform preparation. Compression molding is the most common and lowest-cost process but involves open mold flash that must be trimmed, and the trimming process introduces CS variation.
Surface Quality: ISO 3601-3 Grades N, S, and CS
The N, S, and CS letters are the quality acceptance grades of ISO 3601-3, which classifies surface imperfections — flash, parting-line offset (mismatch), backrind, flow marks, non-fills, and indentations — and sets the maximum acceptable limit for each. The limits scale with cross-section band rather than being single values per grade. Representative Grade N (general purpose) limits, in mm:
| Defect Type | CS 0.80–2.25 | CS 2.25–3.15 | CS 3.15–4.50 | CS 4.50–6.30 | CS 6.30–8.40 |
|---|---|---|---|---|---|
| Off-register / mismatch (offset) | 0.08 | 0.10 | 0.13 | 0.15 | 0.15 |
| Combined flash (offset + flash + parting-line projection), height or width | 0.10 | 0.12 | 0.14 | 0.16 | 0.18 |
| Backrind | 0.18 | 0.27 | 0.36 | 0.53 | 0.70 |
Flash that can be differentiated from the parting line must not exceed 0.07 mm regardless of band. Grade S (special) tightens these limits for applications demanding higher precision or quality — automotive and critical industrial service are typical examples. Grade CS (critical service) is intended for medical and critical aerospace applications where the O-ring surface must be virtually perfect; flash and backrind are not permitted at all at grade CS. Grade N applies by default when the customer does not specify a grade.
Why surface quality interacts with dimensional tolerance: An O-ring with a 0.10 mm flash ridge at the parting line has an effective CS of CS_nominal + 0.10 mm at that location — potentially outside the dimensional CS tolerance even if the base cross-section measures within it. The tighter defect limits of grades S and CS are not cosmetic requirements; they prevent a surface defect from creating a compression anomaly at the defect location.
Tolerance Stack Analysis Worksheet
Use this worksheet to check compression variation for any O-ring groove.
| Parameter | Nominal | Tolerance | Min | Max |
|---|---|---|---|---|
| O-ring CS (mm) | 3.53 | ±0.10 (ISO 3601-1 series G) | 3.43 | 3.63 |
| Groove depth (mm) | 3.04 | ±0.05 | 2.99 | 3.09 |
Calculations
- Min compression = (Min CS − Max depth) / Min CS × 100
= (3.43 − 3.09) / 3.43 × 100 = 9.9%
- Max compression = (Max CS − Min depth) / Max CS × 100
= (3.63 − 2.99) / 3.63 × 100 = 17.6%
Acceptance check: For a dynamic hydraulic seal, target 10–15% compression. The 9.9% minimum is below target. Options:
- Tighten groove depth tolerance to ±0.025 mm
- Negotiate a precision CS tolerance with the molder (e.g. ±0.05 mm)
- Increase nominal compression slightly
Dimensional Inspection Checklist
For incoming O-ring inspection, verify:
- [ ] Quantity matches packing list
- [ ] Material and hardness match specification (check certificate)
- [ ] ID measured at 3 orientations and averaged
- [ ] CS measured at 4 positions and averaged
- [ ] Parting line flash height checked
- [ ] Surface inspected for tears, pits, blisters
- [ ] Tolerance grade confirmed on certificate
- [ ] Date of cure / batch traceability recorded
Cost Impact of Tolerance Grade Selection
| Specification | Manufacturing Cost Multiplier vs. Baseline | Inspection Cost Multiplier | Total Typical Premium |
|---|---|---|---|
| ISO 3601-1 series G / AS568 standard, grade N surface | 1.0× (baseline) | 1.0× | Baseline |
| Series A dimensions or grade S surface quality | 1.2–1.5× | 1.3–2.0× | 30–80% premium |
| Negotiated precision CS tolerance + grade CS surface + documented inspection | 1.5–2.5× | 2.5–5.0× | 100–250% premium |
When the premium is not justified:
- Standard water and pneumatic fittings at low pressure (< 20 bar): series G / grade N is adequate; the groove is forgiving
- Large static seals (ID > 200 mm): The ID variation dominates and CS tolerance has minimal effect — specify series G
- High-volume commodity O-ring positions where standard commercial-grade seals are the market norm
When the premium is clearly justified:
- Vacuum flanges where ±0.05 mm CS variation changes the compression significantly enough to affect helium leak test pass/fail
- Aerospace applications where the failure consequence justifies the documentation and tolerance overhead
- Precision analytical instruments where the gland geometry is very tight and tolerance stack leaves minimal margin
Common Tolerance Mistakes
1. Designing to nominal dimensions without tolerance stack analysis: A groove designed for 15% compression at nominal CS can produce 8% compression (under-squeeze, leak risk) or 22% compression (over-squeeze, high friction) at tolerance extremes with series G O-rings in a ±0.05 mm machined groove. Run the worst-case calculation before signing off on a groove design.
2. Specifying aerospace series or critical-service grades for general hydraulics: Series A dimensions paired with grade CS surface quality typically cost 2–3× a series G / grade N part, with no sealing performance benefit in standard hydraulic cylinder grooves where the groove tolerance (typically ±0.05 mm) dominates the compression stack anyway.
3. Mixing metric and inch O-rings in metric grooves: AS568 inch O-rings have CS values of 1.78, 2.62, 3.53, 5.33, or 6.99 mm — matched to the ISO 3601-1 preferred cross-section values (1.80, 2.65, 3.55, 5.30, 7.00 mm) for practical interchangeability in the CS dimension. However, the ID progressions differ, and an AS568 O-ring installed in a groove designed for the nearest ISO 3601-1 size may have the wrong ID and therefore incorrect stretch or compression for that groove ID.
4. Specifying only CS tolerance without specifying ID tolerance: ID tolerance determines how much the O-ring stretches when installed in a piston groove (piston seal) and therefore affects the actual sealing contact force. A tight CS tolerance paired with a loose ID tolerance still produces variable sealing performance.
5. Not verifying machining process capability against specified groove tolerance: A groove drawing calling for ±0.025 mm depth tolerance means nothing if the manufacturing process has a natural variation of ±0.08 mm. Verify the process Cpk before signing off on a tolerance specification that cannot actually be held in production.
FAQ
Q1: What is ISO 3601-1 and what does it specify?
ISO 3601-1 is the international standard defining inside diameters, cross-sections, dimensional tolerances, and the size identification code for metric O-rings. It publishes two dimensional tolerance tables: series G for general-purpose applications and series A with smaller tolerance limits for aerospace and similar applications (referred to as Class B and Class A in the 2008 edition). The N, S, and CS letters in a full ISO 3601 designation come from a companion standard, ISO 3601-3, which defines surface-quality acceptance grades — they are not dimensional tolerance grades. ISO 3601-1 is the metric counterpart to the AS568 standard for inch-dimensioned O-rings.
Q2: What is AS568 and how does it differ from ISO 3601-1?
AS568 (SAE Aerospace Standard 568) defines standard inch-unit O-ring sizes (ID and CS in inches) identified by dash numbers. The original 1971 issue defined two tolerance classes (Class I and Class II) selected by compound shrinkage rate, but since AS568A (1974) the standard has published a single tolerance table covering all sizes — so there is no "Class 1 vs Class 2" choice in the current standard. The five standard AS568 cross-sections (0.070", 0.103", 0.139", 0.210", 0.275") match the ISO 3601-1 preferred cross-sections (1.78/1.80, 2.62/2.65, 3.53/3.55, 5.33/5.30, 6.99/7.00 mm) closely enough for practical interchangeability in the CS dimension, but the ID progressions use different step sequences.
Q3: What tolerance grade should I use for hydraulic cylinders?
For general industrial hydraulic cylinders at standard operating pressures (below 150 bar), ISO 3601-1 series G or standard AS568 O-rings provide adequate dimensional consistency. The groove machining tolerance (typically ±0.025–0.05 mm for honed cylinder bores) is usually the larger contributor to compression variation than the O-ring CS tolerance. For high-pressure service (> 150 bar) or OEM automotive applications with tight dimensional requirements, series A dimensions or a negotiated precision CS tolerance provide better compression consistency and reduce extrusion risk from CS variation at the high end.
Q4: Do tighter O-ring tolerances always improve sealing reliability?
Not necessarily. Upgrading O-ring tolerance grade improves sealing reliability only when the O-ring dimensional variation is the primary source of compression variation in the overall tolerance stack. If the groove machining tolerance dominates (which is common), tightening the O-ring grade has diminishing returns. The correct approach is to calculate the tolerance stack and identify which component — O-ring CS, groove depth, or bore/rod diameter — contributes the most variation, then address that component first.
Q5: Can I use an AS568 O-ring in a metric groove designed to ISO 3601-1?
For the same nominal cross-section size (e.g., both at 3.53 mm CS), an AS568 O-ring fits the ISO 3601-1 groove CS dimension correctly — the CS values are matched. However, the ID progressions differ between AS568 and ISO 3601-1, so an AS568 O-ring by dash number will often have a different nominal ID than the nearest ISO 3601-1 size for the same groove bore. For a given application, match both the CS and the ID to the groove design — do not substitute an AS568 number for an ISO designation by CS alone without verifying ID.
Q6: What do the ISO 3601-3 grades N, S, and CS mean and when are they required?
N, S, and CS are the surface-quality acceptance grades of ISO 3601-3, not dimensional tolerance grades. Grade N (general purpose) is the default acceptance level. Grade S (special) sets tighter limits on flash, offset, backrind, and other surface imperfections for applications demanding higher precision — automotive and critical industrial service are typical. Grade CS (critical service) requires a virtually perfect surface for medical and critical aerospace applications; flash and backrind are not permitted at all. A fully specified critical seal typically pairs ISO 3601-1 series A dimensions with a grade S or CS surface requirement plus documented inspection. Parts supplied to grade CS with certification typically cost 100–250% more than standard series G / grade N parts — specify that level when the application qualification document explicitly requires it or when the tolerance stack analysis shows that standard tolerances cannot keep worst-case compression within the acceptable range.
Q7: How does parting line flash affect functional tolerance and what limits are acceptable?
Parting line flash is the thin fin of elastomer that forms at the mold parting surface during compression or transfer molding. After trimming or cryogenic deflashing, a residual flash height remains that adds to the effective CS dimension at the flash location. ISO 3601-3 sets flash acceptance limits by grade and by cross-section band: for Grade N, combined flash (offset + flash + parting-line projection) may not exceed 0.10 mm for CS ≤ 2.25 mm, rising to 0.16 mm at CS 4.50–6.30 mm, and flash that can be differentiated from the parting line is capped at 0.07 mm. Grade S limits are tighter, and Grade CS permits no flash at all. In practice, flash height is the most common tolerance nonconformance for small-CS O-rings (1.78–2.62 mm CS), where 0.10 mm flash represents 4–6% of CS — within a Grade N acceptance limit but potentially causing an interference fit at the groove wall when the gland closes. For dynamic seals, flash at the contact face creates a localized stress concentration that can initiate tearing within the first hundred cycles. For gas-tight static seals, flash taller than 0.05 mm at the sealing face can create a radial groove in the contact band that provides a leak path under low-pressure conditions (< 10 bar gas). Accept flash-height inspection certificates — not just a hardness or dimensional check — for grade S and CS applications.
Q8: What dimensional inspection method should I specify for O-ring incoming inspection?
Four methods are used, each suited to different production volumes and tolerance grades. Cone measurement (pin-and-cone or optical cone): the O-ring is placed on a precision cone and ID is read from scale — fast, non-contact, suitable for 100% sampling at high volume; ±0.10 mm accuracy for standard sizes. Optical measurement (vision system or optical comparator): the O-ring is placed on a flat surface and ID, CS, and roundness are measured from the projected image; ±0.05–0.10 mm for ID, ±0.02–0.05 mm for CS; standard where grade S surface acceptance applies. Contact CMM or laser micrometer: for critical-service and aerospace applications; direct CS measurement at multiple points around the circumference; ±0.01–0.02 mm; time-intensive, used for critical parts sampling, not 100% inspection. Attribute gauge (go/no-go fixtures): fast pass/fail check for ID and CS using fixed mandrels and ring gauges; suitable for high-volume acceptance at standard tolerance when documenting process control rather than actual dimensions. For pharmaceutical, aerospace, and other regulated applications, specify contact CMM or optical measurement with a documented AQL (Acceptable Quality Level) sampling plan — typically AQL 1.0 for critical dimensions with 100% visual inspection for surface defects (tears, pits, blisters, flash).
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Need O-rings with specific dimensional tolerances and inspection certificates? Request a quote with your size specification, ISO or AS568 designation, required tolerance series and surface-quality grade, and quantity — we supply O-rings to ISO 3601-1 series G and series A dimensional tolerances with ISO 3601-3 grade N, S, or CS surface quality, complete with dimensional inspection certificates, and can advise on tolerance stack analysis for your groove design.