An oil seal is specified by three caliper-checked dimensions in this order for metric parts: shaft inner diameter (ID) x housing bore outer diameter (OD) x axial width, with common metric sizes including 20x35x7, 30x47x7, and 50x72x8 mm [S1][S2].
Imperial seals reverse the convention and are typically expressed as OD x ID x width in inches, for example 3.000 x 2.125 x 0.375 in, so a technician measuring a worn seal must read the source drawing, not the carcass, to avoid a wrong-size reorder [S3].
How to Measure Three Dimensions Correctly
Published guidance specifies that the shaft must be measured on an unworn section adjacent to the seal path, taking two readings at 90 degrees, because a used lip is heat-set, swollen, or polished and reads off nominal [S2]. A plunge-ground shaft finish of roughly 10 to 20 microinch Ra with no spiral lead is called for, and any existing wear groove requires a wear sleeve before the new seal is fitted [S2].
The bore is checked by pulling the old seal and measuring the housing bore ID after cleaning; the seal OD must match the bore so the outer case presses in cleanly [S1][S2]. For an oil seal, the width is taken from the old part or the bore depth if the seal must sit flush, and any stamped face marking such as 35 52 7 TC NBR gives the size, profile code, and lip elastomer in one line [S2].
Metric vs Imperial Designation Systems
Metric seals conform to ISO 6194 / DIN 3760 patterns, and the designation is always ID x OD x width in millimetres, so 20 47 7 means 20 mm shaft, 47 mm bore, 7 mm wide [S1][S3]. Standard inner diameters cluster between 10 and 30 mm with outer diameters of 22 to 55 mm and widths of 7 to 10 mm, while larger metric stock reaches 100x125x12 mm in commonly held patterns [S5][S2].
Imperial seals use a different format and their numbers are not fractional, so a marking of 300 212 37 translates to a 2 1/8 inch shaft, 3 inch bore, and 3/8 inch width, and an unwary buyer can invert the ID and OD when crossing between the two systems [S3]. If the shaft measures a clean fraction such as 1.000, 1.250, or 1.500 in, the inch collection should be used because near-miss metric sizes are not drop-in substitutes [S2].
Lip Profile Codes: TC, TB, SC, SB and the 2 Suffix
Profile letters carry two pieces of information: the first letter is the lip arrangement (T = spring-loaded plus dust lip, S = single spring-loaded lip, V = single lip with no spring) and the second letter is the outer case (C = rubber-covered OD, B = exposed metal OD), while a trailing 2 such as TB2 or SB2 marks a reinforced heavy-duty case [S2].
The four workhorse profiles line up against duty as follows: TC is rubber-covered with dust lip and garter spring, the default for dirty or outdoor service such as agricultural and construction equipment; TB is metal OD with dust lip, used where a rigid press fit in a clean machined bore is preferred; SC is rubber-covered single lip for clean indoor gearboxes, pumps, and motors, with lower friction and cost; SB is metal OD single lip for clean environments needing a firm metal-to-metal bore [S2][S3]. For static sealing of threaded fasteners, a bonded seal (Dowty type) washer serves a different duty class from a rotary oil seal and is sized by bolt or thread standard, including Metric, BSP, BSPP, BSPT, UNF, and SAE [S4].
Elastomer Selection by Fluid and Temperature
NBR (nitrile) is the default elastomer for hydraulic oil, mineral oil, diesel, and general industrial service because it offers good oil resistance at the lowest cost [S2][S4]. FKM (Viton-class) is specified for high-temperature hydraulic systems, synthetic lubricants, fuel systems, and chemical processing where heat and chemical compatibility dominate the failure modes [S3][S4].
EPDM is the correct choice for water-based hydraulic fluids, coolant systems, and steam service because of its water resistance and weathering behaviour, but it is not compatible with mineral oils and must not be substituted for NBR in oil-lubricated housings [S4]. For high surface speeds, the lip generates frictional heat and the material choice shifts toward FKM or PTFE, often with a special lip geometry and improved lubrication [S3].
Shaft, Pressure, and Speed Operating Envelopes
Recommended shaft surface finish is 0.2 to 0.8 micrometre Ra, because insufficient hardness leads to wear grooves and excessive roughness accelerates lip wear, both of which shorten seal life independent of the seal size [S3]. Shaft surface hardness is therefore a selection input on a par with dimension, and a soft or hardened-but-ground-too-rough shaft will eat a correctly sized seal in short order.
Rotary oil seals are radial lip devices intended for low-to-moderate pressure duty; published guidance reserves them for environments where a thin hydrodynamic oil film can form at the lip, so a static differential pressure beyond roughly 0.05 MPa typically calls for a different sealing class such as a mechanical seal or a bellows seal rather than a reinforced-cased TC or TB radial lip part [S3]. High peripheral speed shifts the material decision because lip heat rises with surface velocity, and high-speed duty usually requires FKM, PTFE, or a specially designed lip geometry with better lube retention [S3].
Who Should Not Pick a Standard TC NBR
A standard rubber-covered TC seal in NBR is the wrong pick for three classes of duty: chemical processing with aggressive fluids above 100 degrees C, where FKM or FFKM is needed [S3][S4]; water and steam systems, where EPDM must be used and NBR will swell and fail [S4]; and high-pressure static differential pressure, where a radial lip seal gives way to a mechanical seal, cartridge seal, or a metal-jacketed gasket rather than a larger TC2 lip [S3].
For threaded fitting sealing on hydraulic adapters, a radial shaft seal is also the wrong product entirely, and a bonded seal matched to BSPP, BSPT, UNF, or SAE thread geometry is the correct fit, with the elastomer element chosen from the same NBR, FKM, EPDM family used for rotary parts [S4]. Buyers specifying gaskets for static flange duty can also consult the broader 2026 cost breakdown at Gasket Price and Cost Guide: 2026 Spec and Budget Breakdown to keep the static-seal budget aligned with the rotary-seal decision.
Shortlist Logic and Verification
The decision flow is fixed: lock the three dimensions first (ID, OD, width), confirm the unit system against the drawing, then pick the profile from the duty (TC for dirty outdoor, SC for clean indoor, TB for clean machined bores, SB for clean metal-to-metal, TB2 / SB2 for heavy-duty), and finally pick the elastomer from the fluid and temperature window (NBR for petroleum oils, FKM for hot or aggressive media, EPDM for water and steam) [S1][S2][S3][S4]. Verification steps include checking the stamped face code, confirming a clean 10 to 20 microinch Ra shaft, and replacing any grooved lip track with a wear sleeve before installation [S2].
The next trackable signal is the revision status of ISO 6194 and DIN 3760 dimensional tables, since metric stock between 10 and 100 mm shaft diameter is widely catalogued to these patterns and any change would shift the cross-reference logic used by every stock chart in this guide [S1][S2].