O-Ring Kit Guide: AS568 Assortments & Custom Sets

An O-ring kit is a multi-size assortment of O-rings in organized storage — the maintenance equivalent of a full wrench set. The right kit means the correct seal is always in the van, the toolroom, or the field pouch when the job requires it. The wrong kit means a 382-piece box where the 12 sizes you actually need are always the first to run out, while 30 other sizes sit untouched for years.

Quick answer: For general petroleum/hydraulic service: standard AS568 NBR 70 Shore A kit covering -004 to -284 (382 pieces) handles the majority of industrial maintenance. For chemical, food, or pharmaceutical service: material-specific FKM, EPDM, or VMQ FDA-grade kit is required — NBR is incompatible. For known equipment fleets: custom kits with your actual size list and consumption-weighted quantities eliminate wasted inventory. Key decision factors: (1) match kit material to system fluid; (2) weight quantities toward sizes actually used (most facilities use 15–20 sizes regularly); (3) for high-pressure positions (> 150 bar), include 90 Shore A variants; (4) for regulated applications, kit compounds must carry compound-specific FDA/NSF/USP certification, not just a generic supplier quality system certificate. Selecting the right kit requires knowing the fluid media (which determines material), the equipment size range (which determines size distribution), the usage rate (which determines quantities per size), and the compliance requirements (FDA, NSF/ANSI 61, aerospace spec) that constrain which compounds are acceptable.

Types of O-Ring Kits

Standard AS568 Assortment Kits

The most widely available type. Contains sizes spanning the AS568 standard inch-unit dimensions, typically covering the most commonly used dash numbers in a single material.

Typical standard AS568 NBR kit contents:

• Material: 70 Shore A black NBR, 33–36% acrylonitrile content (general-purpose grade)
• Size range: -004 through -284 (covering 1.78 mm, 2.62 mm, and 3.53 mm CS series, plus portions of the 5.33 mm series)
• Piece count: 170-piece (partial range) to 382-piece (full range)
• Quantity per size: typically 1–5 pieces per size, with more pieces in smaller sizes
• Storage: labeled compartment case, typically polypropylene with hinged lid

What standard AS568 kits do not include:

• FKM, EPDM, or silicone material (NBR only)
• Large -4XX or -5XX dash numbers (6.99 mm and 8.74 mm CS) — usually sold separately
• 90 Shore A compound for high-pressure extrusion resistance
• Specialty compounds (FDA, NSF, USP Class VI, aerospace)

The standard kit limitation: For facilities handling petroleum hydraulics in standard industrial conditions, a 382-piece NBR kit covers the vast majority of replacements. For any facility with chemical-resistant requirements, steam, food contact, or specialty fluid service, standard NBR is the wrong material for a significant portion of equipment.

Material-Specific Kits

Single-material kits covering the same AS568 size range in a specific elastomer. Used where all equipment in the facility uses the same fluid type or where the facility has identified that one material covers its full equipment range.

Dual-material kits: Facilities where most positions are NBR but a defined set of high-temperature or chemical-resistant positions require FKM can specify a split kit — for example, -2XX and -3XX sizes in NBR for hydraulic circuits with FKM versions of the same sizes for chemical-service positions. This eliminates the risk of installing the wrong material by keeping them in separate, labeled compartments.

Metric Kits (ISO 3601)

Kits based on ISO 3601-1 metric sizes rather than AS568 inch dimensions. Standard for European-manufactured equipment and required where equipment drawings specify metric O-ring sizes.

Key ISO 3601 metric series commonly stocked in metric kits:

• CS 1.78 mm: ID range 1.8–6.0 mm (small pneumatic, instrumentation)
• CS 2.62 mm: ID range 3.0–40.0 mm (light hydraulics, fittings)
• CS 3.53 mm: ID range 6.0–100.0 mm (standard hydraulic cylinders)
• CS 5.33 mm: ID range 10.0–150.0 mm (larger hydraulic equipment)
• CS 6.99 mm: ID range 50.0–200.0 mm (large bore hydraulics, manifolds)

Metric vs inch size confusion: The six standard CS values are shared between AS568 and ISO 3601 (1.78, 2.62, 3.53, 5.33, 6.99, 8.74 mm), but the ID progressions are completely different. An AS568 -214 (ID = 1.000" = 25.40 mm, CS = 3.53 mm) does not correspond to any standard ISO 3601 size with the same ID. Always specify O-rings by the system used in the original equipment drawing — mixing systems causes installation of incorrectly sized seals.

Application-Specific Kits

Pre-selected kits designed for a specific equipment type or service environment, with size distribution and quantity biased toward what that application actually uses:

Custom Kits

Built entirely to a facility-specific specification: sizes, quantities per size, material, hardness, compound grade, and storage format. Custom kits are the most efficient solution for facilities with a defined equipment fleet.

When a custom kit outperforms a standard kit:

• The equipment fleet uses a known set of O-ring sizes — a standard 382-piece kit includes 80+ sizes that will never be needed
• Repair frequency is known for specific positions — some sizes need 10+ pieces on hand; others need 2
• Multiple materials are required — a single-material standard kit covers only part of the fleet
• Special certifications are required — FDA, NSF, aerospace

Kit Type Comparison

How to Select the Right Kit

Step 1: Identify Your Fluid Media

The system fluid determines material compatibility. Specifying the wrong material causes seal failure, system leakage, and in some cases fluid contamination:

Step 2: Identify Your Size Range and Distribution

Review your equipment and identify which O-ring sizes are actually in service. Standard 382-piece AS568 kits are distributed roughly equally across all sizes — but most facilities use 15–20 sizes heavily and rarely or never use the other 60+.

Size distribution by equipment type:

Identify your most-used sizes by pulling maintenance records or spare parts consumption data for 12 months. Order 3–6 months of consumption per size in each kit replenishment.

Step 3: Determine Quantity Requirements

Quantity per size in a kit determines how many repairs the kit covers before restocking. The correct quantity depends on:

• Failure frequency by size (some sizes fail weekly; others fail once per year)
• Criticality of the equipment (critical production lines need more stock than secondary equipment)
• Restocking interval (field service requires more stock per kit than a workshop that orders weekly)

Practical rule: For the 10 most frequently replaced sizes, carry 5–10 pieces per kit. For less frequent sizes, 2–3 pieces is adequate. For rarely used large sizes, 1 piece is sufficient.

Step 4: Hardness Requirements

Standard kits contain 70 Shore A — correct for general industrial service at pressures below 100 bar. For high-pressure hydraulic and pneumatic service:

For facilities with high-pressure equipment, a kit containing both 70 Shore A and 90 Shore A versions of the common sizes eliminates the need to carry two separate kits.

Step 5: Compliance and Certification Requirements

What to Look for in a Quality Kit

A quality O-ring kit is not just a sorted box of O-rings — the quality of the compound and the accuracy of the sizes determines whether the kit actually fixes the problem or creates a new one:

Material quality indicators:

• Compound certificate available: The supplier should be able to provide the compound designation, acrylonitrile content (for NBR), and cure system
• Correct ACN content for NBR: General-purpose NBR kits should use 33–36% ACN — this provides the standard balance of oil resistance and low-temperature performance. Very low ACN (< 28%) is more flexible but less oil-resistant; very high ACN (> 40%) has poor low-temperature performance and is not appropriate for general kits
• Dimensional accuracy: O-rings manufactured to ISO 3601-1 Grade N or AS568 Class 1 tolerances, not simply "to nominal" without measurement

Storage quality indicators:

• Labeled compartments with dash number or size printed on each compartment lid
• Size index printed inside the case lid or included as a laminated reference card
• Durable, sealed case construction that keeps dust and moisture out during storage
• O-rings individually bagged or in compartments that prevent contact between sizes

Shelf life indicators:

• Manufacture date or lot code on packaging (required to track against shelf life limits)
• For NBR: 10–15 years shelf life from manufacture date under correct conditions (< 25°C, < 65% RH, away from ozone and UV)
• For FKM: rated as unlimited per SAE AS5316 — practical inspection period 20–30 years

Building a Custom O-Ring Kit

For facilities with a defined equipment fleet, a custom kit eliminates unused sizes, provides the correct quantities of high-turnover sizes, and can combine multiple materials in one organized storage container.

5-step custom kit design process:

1. List all sealing positions across the maintained equipment fleet — use maintenance manuals or equipment drawings to identify every O-ring size
2. Identify size and material for each position — cross-reference with system fluid and temperature to verify correct material
3. Estimate annual consumption per size based on 12 months of maintenance records or an estimate of failure frequency × number of equipment units
4. Calculate kit quantities based on 3–6 months of consumption per size (a 6-month supply is suitable for field service; 3-month for workshop with reliable supply chain)
5. Select storage format — choose a case with compartment count ≥ the number of distinct sizes, with compartment depth suitable for the quantities required

Custom kit specification checklist:

FAQ

Q1: How many O-rings are in a standard AS568 kit?

Standard AS568 kits range from 170 pieces (partial range, typically -004 to -210) to 382 pieces (full range, -004 to -284 with some -3XX sizes). Piece count per size varies by kit — a 382-piece kit at 1 piece per size covers 382 different sizes; a 382-piece kit at 3 pieces per size covers approximately 127 sizes with 3 units each. For maintenance applications, a kit weighted toward more pieces of the commonly used -2XX and -3XX sizes is more practical than a single-piece-per-size full-range kit.

Q2: Can I get a kit with both NBR and FKM?

Yes. A dual-material custom kit is practical for facilities where most positions are NBR but a specific set of chemical or high-temperature positions require FKM. Specify which sizes you need in each material, and the storage case can be organized with a divider or color-coded compartments to prevent mixing. This is more cost-effective than carrying two complete kits when only a subset of positions differs.

Q3: What is the shelf life of O-rings in a kit?

Under correct storage conditions (< +25°C, < 65% relative humidity, away from UV light, away from ozone-generating equipment like motors and fluorescent lamps): NBR O-rings in kits have a 10–15 year shelf life; FKM has rated unlimited shelf life (SAE AS5316); EPDM 10–15 years; VMQ 15–20 years; polyurethane 3–5 years. Mark the kit with its manufacture date and rotate stock (use oldest first). Annual inspection is recommended — inspect for cracking, hardening, or surface degradation before using O-rings from kits stored more than 5 years.

Q4: Do kit O-rings have the same quality as individually ordered O-rings?

They should — and from a quality supplier, they do. Both use the same compound and manufacturing process. The distinction is that kit O-rings are often a standard warehouse compound selected for breadth of compatibility, while individually ordered O-rings can be specified to a precise compound and certification. For critical applications (aerospace, pharmaceutical, high-pressure) that require specific compound certifications, order O-rings individually with compound certificates rather than relying on a generic kit.

Q5: Can I build a kit for a specific machine model?

Yes. If you provide the make, model, year, and service manual of your equipment, we can cross-reference the sealing positions and build a kit containing exactly the sizes and materials required for that machine — including backup rings, any specialty compounds, and the installation tools needed for that equipment. Machine-specific kits are particularly practical for OEM service dealers, field service organizations maintaining a single equipment line, and facilities with a fleet of identical machines.

Q6: What is the correct storage condition for a kit in a field service van?

A field kit stored in a vehicle faces more severe conditions than a warehouse kit — temperature extremes (summer heat reaching +60°C in a parked vehicle, winter cold below −20°C) and UV exposure from windows. To extend shelf life: (1) store the kit in a sealed case away from direct sunlight; (2) if possible, store in a climate-controlled vehicle storage compartment; (3) inspect the kit contents annually for cracking, hardening, or surface degradation; (4) rotate kit stock annually — rotate old kit into the shop/warehouse for continued use and replenish the field kit with fresh stock. NBR exposed to sustained summer heat degrades at approximately 4× the rate of warehouse-stored NBR.

Q7: What is the difference between an O-ring kit and an O-ring assortment?

The terms are used interchangeably in the industry. Both describe a multi-size collection of O-rings in organized storage. "Kit" often implies a specific-purpose collection (hydraulic repair kit, plumbing kit), while "assortment" often implies a general-purpose breadth-of-coverage collection. The distinction is not standardized — evaluate any kit or assortment on its specific contents (sizes, quantities, materials, compound certification) rather than the label.

Q8: How do I determine the correct quantities per size for a custom kit to minimize both stockout and excess inventory?

The most reliable method is a 12-month consumption analysis from maintenance records: count how many O-rings of each size were replaced across all equipment over the previous year, then size the kit to carry 3 months of consumption per size (enough buffer for two restocking cycles with normal delivery lead times). Where maintenance records are not available, use an equipment-count and failure-rate estimate: multiply the number of each equipment unit by the estimated annual failure rate per seal position (typically 0.5–2 failures per position per year for dynamic seals in industrial equipment, 0.1–0.5 for static seals) and divide by 4 for a 3-month kit quantity. Apply two adjustments: (1) for sizes used in critical equipment where a stockout causes production loss, add 50–100% buffer stock; (2) for sizes used only in rarely maintained or low-consequence equipment, carry the minimum (1–2 pieces). After the first year of use, update the quantities based on actual consumption — a well-designed custom kit is reviewed annually and adjusted to match observed repair frequencies rather than initial estimates.

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Need a custom O-ring kit for your equipment fleet? Contact our engineering team with your equipment list, fluid media, size requirements, and quantity estimates — we design and supply custom kits from 1-piece MOQ per size with material certificates, sized for your specific maintenance cycle and delivered in labeled, organized storage cases.