Seals & Gaskets Guide

A complete guide to seals and gaskets: O-Ring size charts (AS568/ISO 3601), seal material selection (NBR, FKM, EPDM, PTFE), seal types, mechanical face seals, flange gaskets, O-Ring groove design, and a squeeze/fill calculator.

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Seals & Gaskets

O-Ring · Mechanical · Lip Seal · Gasket · Materials · Groove Design · Calculator

🔵 O-Ring — Sizes and Principles

AS568 — Common O-Ring Sizes

ANSI/AS568Common in the US

AS568	ID (mm)	Cord d₂	AS568	ID (mm)	Cord d₂
-006	2.90	1.78	-210	13.87	3.53
-008	4.48	1.78	-214	19.18	3.53
-010	6.07	1.78	-220	28.17	3.53
-012	7.65	1.78	-225	38.00	3.53
-014	9.25	1.78	-309	22.23	5.33
-109	8.74	2.62	-316	36.17	5.33
-116	22.23	2.62	-325	58.42	5.33
-125	41.28	2.62	-330	64.77	5.33
-130	53.34	2.62	-425	88.57	6.99
-145	88.62	2.62	-432	107.32	6.99
-206	6.07	3.53	-440	125.07	6.99
-208	8.74	3.53	-450	150.57	6.99

▸ Outer diameter OD = ID + 2×d₂
▸ -1xx = d₂=1.78mm · -2xx = d₂=3.53mm · -3xx = d₂=5.33mm · -4xx = d₂=6.99mm

ISO 3601 — Metric Sizes

ISO 3601European

Size	ID (mm)	Cord d₂ (mm)	Use
6×1.5	6.0	1.5	Small general
8×1.5	8.0	1.5	General
10×1.5	10.0	1.5	General
10×2	10.0	2.0	General
12×2	12.0	2.0	Common
16×2	16.0	2.0	Common
20×2.5	20.0	2.5	Common
25×3	25.0	3.0	Common
30×3	30.0	3.0	Common
40×4	40.0	4.0	Large
50×5	50.0	5.0	Large
100×5.5	100.0	5.5	Very large

▸ ISO 3601-1: sizes. ISO 3601-3: tolerances. ISO 3601-4: materials
▸ Metric is more common than AS568 outside the US

Squeeze

Critical!

Application Type	Squeeze %	Fill %	Note
Static — internal	15–25%	65–80%	Most common
Static — external	15–25%	65–80%	Face seal
Dynamic axial	10–20%	55–70%	Piston, rod
Dynamic rotary	5–15%	50–65%	↓friction
Cryogenic	20–30%	70–85%	Shrinkage in cold
High temperature	20–25%	70–85%	Compression set

FormulaSqueeze% = (d₂ - h) / d₂ × 100

Fill%= π×d₂²/(4×h×w) × 100

▸ Too little squeeze = leakage. Too much = compression set = leakage!
▸ Always check Fill — the O-ring needs room to expand under pressure

▶ YouTube

Stretch

Ideal stretch1–5% — an O-ring slightly larger than the groove

Maximum6% — above this = ↓cord diameter = ↓squeeze!

CompressionUp to 3% (O-ring larger than the groove)

FormulaStretch% = (Groove ID − O-ring ID) / O-ring ID × 100

▸ A stretched O-ring = a smaller cord diameter = less squeeze!
▸ Choose an O-ring with an ID slightly smaller than the groove — 1-3% stretch

🧪 Seal Materials — Selection by Application

Seal Material Comparison — Full Table

Material	Abbr.	Range °C	Mineral Oil	Fuel	Water/Steam	Acids	Bases	Cryogenic	Use	Price
Buna-N / Nitrile	NBR	−40 / +120	✅ Excellent	✅ Good	✅	⚠️	⚠️	❌	Hydraulics, oil, most common	★
Viton	FKM	−20 / +200	✅	✅ Excellent	⚠️	✅	⚠️	❌	Defense, chemical, high temp	★★★
EPDM	EPDM	−50 / +150	❌	❌	✅ Excellent	✅	✅	⚠️	Water, steam, brakes — not for oil!	★
Silicone	VMQ	−60 / +220	⚠️	⚠️	✅	⚠️	⚠️	⚠️	Food, medical, extreme temp	★★
PTFE	PTFE	−200 / +260	✅	✅	✅	✅ Excellent	✅ Excellent	✅	Aggressive chemicals, universal	★★★
Neoprene	CR	−40 / +120	⚠️	⚠️	✅	⚠️	⚠️	⚠️	Air, freon, outdoor environment	★
HNBR	HNBR	−30 / +150	✅	✅	✅	⚠️	⚠️	❌	Improved NBR — automotive, H₂S oil	★★
Kalrez	FFKM	−20 / +315	✅	✅	✅	✅	✅	⚠️	All chemicals — very expensive!	★★★★★
Polyurethane	AU/EU	−30 / +100	✅	⚠️	⚠️	❌	❌	❌	U-Cup, wiper — mechanical strength	★★
Natural Rubber	NR	−50 / +80	❌	❌	✅	⚠️	⚠️	⚠️	Water, air, vibration	★
Butyl	IIR	−50 / +120	❌	❌	✅	✅	✅	⚠️	Gas sealing — tubes, reservoirs	★
Viton GF	FKM-GF	−40 / +200	✅	✅	⚠️	✅	⚠️	✅	Cryogenic Viton — LNG, LN₂	★★★★

▸ Golden rule: NBR = the first to try (cheap + oil-resistant). If unsuitable ← FKM
▸ EPDM + oil = ❌ immediate! Swelling and failure
▸ Cryogenics: PTFE (excellent), Viton-GLT/GF (good), NBR (❌)

Shore A Hardness — Effect on Performance

Shore A	Characteristic	Max Pressure	Use
40–50	Very soft	10 bar	Delicate sealing, light contact
60–70	Standard	70 bar	General static — ← most common
70–80	Medium hard	200 bar	Dynamic, moderate pressures
80–90	Hard	400 bar	High pressures + backup
90+	Very hard	700 bar+	With a backup ring only

▸ Shore 70 = the default for most applications
▸ High pressure → higher Shore → ↓extrusion
▸ Maximum extrusion gap: 0.05–0.1mm at Shore 70

Compression Set

DefinitionPercent of permanent set after sustained load — the O-ring doesn't return to its shape

FormulaCS% = (d₀ − d₁) / (d₀ − d_spacer) × 100

NBR 70CS ≈ 20% after 70°C/22h

FKMCS ≈ 15% after 175°C/22h

SiliconeCS ≈ 30–50% — relatively high!

PTFECS ≈ 60–70% — cold flow!

▸ High CS = the O-ring "stays flat" ← ↓squeeze ← leakage
▸ High temperature = ↑CS faster
▸ Kalrez CS = 5–15% — the best

▶ YouTube

⚙️ Seal Types — All Types

O-Ring

Most CommonStatic+Dynamic

Max pressureStatic: 700 bar · Dynamic: 100 bar (no backup)

SpeedUp to 0.5 m/s axial · up to 2 m/s rotary

AdvantagesCheap, available, easy to install, bidirectional

DisadvantagesExtrusion at high pressure, spiral failure in dynamic use

▶ YouTube

X-Ring / Quad-Ring

Dynamic

Principle4 lips instead of 2 — ↓spiral failure in dynamic use

PressureUp to 400 bar

Advantage over O-ring↓friction, ↓torsion, ↑dynamic durability

DisadvantageMore expensive, less available

UseHydraulic piston, rod, reciprocating

Backup Ring

High Pressure

PrincipleA PTFE/nylon ring on the pressure side — prevents extrusion

Pressure with backupUp to 700 bar!

Single BackupUnidirectional pressure — backup on the pressure side

Double BackupBidirectional pressure — backup on both sides

MaterialPTFE (spiral-wound), nylon, polyester

▸ Extrusion gap: ≥70 bar → backup mandatory!
▸ Maximum gap: 0.05–0.1mm depending on Shore

U-Cup / Lip Seal

Piston/Rod

PrincipleA flexible lip pressed by pressure — self-energizing

PressureUp to 500 bar

SpeedUp to 1 m/s (polyurethane)

DirectionalityUnidirectional! Orient toward the pressure

MaterialPolyurethane, NBR, FKM, PTFE

UseHydraulic cylinders, pneumatic, actuators

▶ YouTube

Wiper / Scraper Seal

Rod Cleaning

PrincipleScrapes dirt off the rod as it enters the cylinder

PositionOutermost — the cylinder head

MaterialPolyurethane, NBR, PTFE

Failure without itDirt enters → scratches chrome plating → leakage!

▸ Wiper + rod seal = a complete rod protection set
▸ Always replace the wiper when replacing the rod seal

V-Ring Seal

Rotating Shaft

PrincipleA rubber ring pressed against the shaft shoulder — self-clamping

PressureLow — not for sealing under pressure

SpeedUp to 12 m/s

UseProtecting grease-lubricated bearings + dirt barrier

AdvantageNo groove needed! Installs directly on the shaft

ManufacturersSKF V-Ring, Freudenberg

PTFE Lip Seal / Rotary Shaft

High Speed

PrincipleA PTFE lip on a rotating shaft — ↓friction

SpeedUp to 20 m/s! (vs. NBR 6 m/s)

Temperature−200 to +260°C

UseThermal shaft, high-speed pumps, chemical

Shaft RaRa 0.2–0.8μm — must pay attention!

Metal C-Ring / E-Ring

ExtremeTemp+Pressure

PrincipleA metal ring (SS/Inconel) that compresses elastically

PressureUp to 700 bar

TemperatureUp to 600°C (Inconel)

UseJet engines, aerospace, nuclear reactors

CostVery high — used only in critical applications

▶ YouTube

🌀 Mechanical Face Seal

Operating Principle

Zero LeakagePumps

Principle2 lapped faces (primary/mating ring) pressed against each other — ↓leakage

Primary RingA ring rotating with the shaft (carbon/SiC/WC)

Mating RingA stationary ring in the pump housing (SiC/SS/ceramic)

SpringPresses the primary ring against the mating ring

Allowed leakage3–5 drops/hour — a mechanical seal isn't absolute zero leakage

Durability10,000–50,000 operating hours

▸ A thin liquid film between the faces = enables free rotation + ↓wear
▸ Never run dry! → immediate failure
▸ Flushing Plan (API 682) = cools + cleans the faces

▶ YouTube

Face Materials

Pair	Primary	Mating	Use
Common	Carbon	SiC	Water, coolant, most fluids
Hard	SiC	SiC	Abrasive fluids
Chemical	Carbon	Alumina	Chemicals, acids
High temp	WC	SiC	Temperature + abrasive

▸ Never carbon-carbon! ← ↑wear, ↑heat
▸ SiC vs. SiC = for fluids with solids (slurry)

Mechanical Seal Types

Single SealOne face pair — ← most common. Minor leakage to atmosphere

Double SealTwo face pairs + barrier fluid — zero leakage to atmosphere

TandemTwo face pairs + quench — backup for failure

CartridgeA pre-assembled unit — ↓installation errors

API 682The oil industry standard — Plans 11/13/23/53A/53B

Mechanical Seal Failures — Diagnosis

Dry RunningScorching, chipping, sudden failure — ↑↑common!

CavitationFace erosion from bubbles — ↑flow, ↓NPSH

MisalignmentUneven wear — check pump alignment

CrystallizationSalts crystallizing on the faces — ↑flushing

CokingHot fluids hardening — ↑quench

📋 Gaskets — Flange and Joint Seals

Gasket Types

Type	Material	Pressure	Temp.	Use	Standard
Spiral Wound	SS/Graphite	Up to 250 bar	Up to 800°C	Chemical piping, oil	ASME B16.20
Ring Joint (RTJ)	SS/Iron/Monel	Up to 700 bar!	Up to 650°C	HP piping, coal	ASME B16.20
Kammprofile	SS+Graphite	Up to 200 bar	Up to 600°C	Special flanges	ASME B16.20
Full Face Flat	Rubber/PTFE	Up to 20 bar	Up to 150°C	Cast iron flanges	ASME B16.21
Raised Face (RF)	Spiral/Graphite	Up to 250 bar	Up to 500°C	SS/CS flanges	ASME B16.5
Metal Jacketed	SS/Al+Fiber	Up to 100 bar	Up to 400°C	Heat exchangers	—
Graphite Sheet	Flexible graphite	Up to 100 bar	Up to 450°C	Chemical, steam	—
PTFE Sheet	Virgin PTFE	Up to 25 bar	Up to 200°C	Aggressive chemicals	—
Compressed Fiber	Aramid/glass	Up to 50 bar	Up to 300°C	General, water, air	—

▶ YouTube

Spiral Wound Gasket — Details

Very Common

StructureV-shaped metal strip + filler wound in a spiral

Inner RingMarked yellow — SS304/316

FillerGraphite (general) / PTFE (chemical)

WindingSS304 / SS316 / Inconel

Color codingGreen=graphite · white=PTFE · blue=mica

Seating StressMinimum: 69 MPa (ASME)

▸ Inner ring is mandatory! Prevents buckling under vacuum
▸ Storage: flat + dry — don't compress!

▶ YouTube

Ring Type Joint (RTJ)

High Pressure

PrincipleA soft metal ring in a groove — ← pressure clamps it tighter

Shape ROval — relatively low pressures

Shape RXPressure energized — ← self-clamping

Shape BXSuper high pressure — subsea, wellhead

MaterialMust be softer than the flange! Iron < CS < SS < Monel

▸ RTJ is single-use! Never reuse
▸ The groove must be clean and free of defects

▶ YouTube

Flange Facing

RF (Raised Face)Ra 3.2–6.3μm · Serrated finish (ASME B16.5)

FF (Full Face)Cast iron flanges — full face gasket

RTJ GrooveRa 0.8μm — polished! For RTJ

TG (Tongue & Groove)↑gasket retention — precise

MF (Male/Female)Male/female — gasket held in place

▸ Serrated finish: 125–250 AARH = Ra 3.2–6.3μm
▸ Stock finish: Ra 12.5μm — not suitable for a spiral-wound gasket!

▶ YouTube

📐 O-Ring Groove Design — per ISO 3601 and Parker

Groove Dimensions — Piston Seal (Internal)

Cord d₂	Groove Depth h	Groove Width w	Corner R	Ra Required	Squeeze%
1.5 mm	1.15–1.20	2.0–2.2	0.2–0.4	Ra 0.4–0.8	15–22%
1.78 mm	1.37–1.42	2.4–2.6	0.2–0.4	Ra 0.4–0.8	15–22%
2.0 mm	1.55–1.60	2.7–2.9	0.2–0.4	Ra 0.4–0.8	15–22%
2.62 mm	2.05–2.10	3.5–3.8	0.4–0.8	Ra 0.4–0.8	15–22%
3.0 mm	2.35–2.40	4.0–4.3	0.4–0.8	Ra 0.4–0.8	15–22%
3.53 mm	2.75–2.85	4.7–5.1	0.4–0.8	Ra 0.4–0.8	15–22%
4.0 mm	3.15–3.25	5.4–5.7	0.4–0.8	Ra 0.4–0.8	15–22%
5.0 mm	3.90–4.05	6.7–7.2	0.8–1.2	Ra 0.4–0.8	15–22%
5.33 mm	4.15–4.30	7.1–7.6	0.8–1.2	Ra 0.4–0.8	15–22%
6.99 mm	5.45–5.65	9.4–10.0	0.8–1.2	Ra 0.4–0.8	15–22%

▸ Groove surface Ra: 0.4–0.8μm (static) · 0.2–0.4μm (dynamic)
▸ Corner R is mandatory! A sharp corner = cuts the O-ring
▸ Lay: ⊥ to the offset direction — reduces leakage

▶ YouTube

Face Seal — Groove for Face Sealing

Squeeze20–30% — higher than a piston seal

DimensionsLike piston but h smaller by 10–15%

Surface RaRa 0.4–1.6μm — less critical than dynamic

Dovetail GrooveHolds the O-ring during disassembly — for vertical faces

▶ YouTube

Required Ra on Shaft / Cylinder

Application	Ra (μm)	Method
Dynamic shaft (rod seal)	0.1–0.4	Grinding
Hydraulic cylinder (bore)	0.2–0.8	Honing
Piston (piston seal)	0.2–0.8	Honing
Face seal (static)	0.4–1.6	Milling/turning
Rotary shaft seal	0.2–0.5	Grinding
O-Ring groove (dynamic)	0.4–0.8	Finish turning
O-Ring groove (static)	0.8–1.6	Turning

🧮 O-Ring Seat Calculator

Squeeze and Fill Calculator

Enter O-Ring and groove data for automatic calculation

Cord Diameter d₂ (mm)

Groove Depth h (mm)

Groove Width w (mm)

Application Type

O-Ring Inner Diameter ID (mm)

Groove Diameter ID (mm)

—

Quick Reference Table

d₂	Static Squeeze	Recommended h
1.78	15–25%	1.37–1.52
2.62	15–25%	2.05–2.23
3.53	15–25%	2.75–2.99
5.33	15–25%	4.15–4.53
6.99	15–25%	5.45–5.94

▸ Squeeze = (d₂ − h) / d₂ × 100
▸ Fill = π×d₂² / (4×h×w) × 100
▸ Stretch = (Groove ID − OR ID) / OR ID × 100

📋 Standards and Specifications — Seals

O-Ring Standards

AS568ANSI — O-Ring sizes (common in the US)

ISO 3601Metric — sizes + tolerances + materials

BS 1806British (superseded by ISO 3601)

DIN 3771German (superseded by ISO 3601)

MIL-G-5514Defense — critical O-Rings

Parker O-Ring HandbookORD 5700 — a complete design guide

Flange Gasket Standards

ASME B16.20Spiral Wound + Ring Joint Gaskets

ASME B16.21Gaskets for Flanged Joints

EN 1514Flanges and their joints — gasket dimensions

ASME B16.5Pipe Flanges (including facing finish)

Mechanical Seal Standards

API 682Shaft Sealing Systems — Pumps (Oil & Gas)

API 610Centrifugal Pumps — Seal Requirements

ISO 21049Pumps Shaft Sealing Systems (=API 682)

EN 12756Mechanical Seals — Dimensions

ManufacturersJohn Crane, Flowserve, EagleBurgmann, Burgmann

Seal Material Standards

ISO 1629Rubber — Nomenclature (NBR, FKM, EPDM...)

ASTM D2000Classification of Rubber Materials

SAE J200= ASTM D2000 (defense/automotive)

AMS 7276FKM O-Ring Compounds (aerospace)

FDA 21 CFRFood Grade Elastomers

USP Class VIMedical Grade — biocompatibility