Choosing the correct replacement oil seal starts with accurate measurements. Oil seal dimensions are normally written in the following order:
Inner diameter × outer diameter × width
For example, an oil seal marked 30×42×7mm is designed for a 30mm shaft, a 42mm housing bore and has a width of 7mm.
This guide explains how to identify and measure an oil seal correctly before ordering a replacement.
What Do ID, OD and Width Mean?
- ID – Inner Diameter: The nominal diameter of the shaft on which the oil seal operates.
- OD – Outer Diameter: The diameter of the seal’s outside surface, which fits into the housing bore.
- Width: The overall thickness of the oil seal from one face to the other.
These three measurements are the main information required when searching for a replacement rotary shaft seal.
Tools Required
For the most accurate results, use a digital or vernier caliper. A ruler or tape measure may provide an approximate measurement, but it is usually not accurate enough for identifying smaller oil seals.
Before measuring, clean the seal, shaft and housing carefully so that dirt, grease or damaged material does not affect the result.
Step 1: Check the Existing Seal Markings
First, inspect both faces of the existing oil seal. The size, material or manufacturer’s reference may be moulded or printed on the seal.
A marking such as 30 42 7, 30×42×7 or 30-42-7 normally represents:
- 30mm inner diameter
- 42mm outer diameter
- 7mm width
If the markings are clear and the original seal is correct for the application, they are usually the best starting point for identifying a replacement.
Step 2: Measure the Shaft or Inner Diameter
Where possible, measure the shaft directly with a caliper. Place the caliper jaws across the shaft and record the diameter in millimetres.
Measuring the inner sealing lip of a used oil seal may not provide an accurate nominal size. The lip is designed to grip the shaft and may also have become worn, stretched or damaged during service or removal. For this reason, measuring the shaft is generally more reliable.
If the shaft cannot be measured, take several measurements across the seal opening and compare the result with common standard sizes.
Step 3: Measure the Outer Diameter
Measure across the widest outside edges of the oil seal. Keep the caliper straight and avoid measuring over distorted or damaged areas.
If the outside surface is rubber-covered, take several measurements around the seal and use the most consistent result. When possible, also measure the housing bore into which the seal fits.
Step 4: Measure the Seal Width
Measure the overall thickness of the seal from one flat face to the other. Do not include any flexible lip that projects beyond the main body unless it is part of the original specified profile.
Some seals may have a stepped, flanged or specialist design, so photographs and additional dimensions may be required.
Identify the Oil Seal Profile
Dimensions alone do not always identify the complete seal. You should also check its construction and lip arrangement.
A TC oil seal normally has a spring-loaded primary sealing lip and a secondary dust lip. The primary lip helps retain oil or grease around a rotating shaft, while the dust lip helps reduce the entry of dirt and other external contaminants.
Other seal profiles may have a single lip, metal outer case, pressure-resistant construction or additional sealing features. If you are uncertain, send clear photographs of the front, back and side of the original seal.
Check the Seal Material and Application
The replacement seal must also be suitable for the operating environment. Important factors include:
- Fluid or lubricant being sealed
- Operating temperature
- Shaft speed
- Pressure
- Shaft condition and surface finish
- Exposure to dust, water, mud or chemicals
Our current UK-stock TC oil seal range is manufactured from NBR (nitrile rubber). Other materials, including FKM, may be available through our manufacturing partner network for applications requiring different temperature or chemical resistance.
For a detailed comparison of these materials, read our NBR vs FKM Oil Seals guide, covering temperature ranges, chemical resistance, typical applications and important selection factors.
Common Oil Seal Measuring Mistakes
- Measuring only the worn inner sealing lip instead of the shaft
- Confusing inner diameter with outer diameter
- Rounding measurements too far from a standard size
- Ignoring the seal width or lip profile
- Ordering by dimensions without checking material and operating conditions
- Measuring a distorted seal immediately after removal
Need Help Identifying an Oil Seal?
MAV Bearings Ltd holds more than 90 sizes of TC double-lip NBR oil seals in UK stock for fast dispatch. You can browse our available range below:
Browse TC Oil Seals in UK Stock
If your required size is not listed, send us the dimensions, required quantity, application details and clear photographs of the existing seal. We can check our UK stock and manufacturing partner network for a suitable option.
Always verify the required dimensions, material, seal profile and operating conditions before ordering or installing a replacement oil seal.