How to Measure an Oil Seal Correctly

Selecting the correct oil seal requires more than matching its dimensions. The seal design, material, installation direction, and operating conditions must also be considered. An incorrect seal may cause leakage, premature wear, or damage to the shaft and housing.

Oil seal dimensions are normally written as:

ID × OD × Width

Example:

35 × 52 × 7 mm

• 35 mm = Inside diameter or shaft diameter

• 52 mm = Outside diameter or housing bore diameter

• 7 mm = Seal width or thickness

Recommended Measuring Tools

Use a vernier caliper, digital caliper, or micrometer for accurate measurement. A ruler or measuring tape is not recommended because it may not provide sufficient accuracy.

Before measuring, clean the seal and check for any visible information, such as the brand, part number, dimensions, seal type, and material.

1. Measure the Shaft Diameter — ID

The inside diameter should be determined from the actual shaft rather than from the opening of the used seal lip. The sealing lip is normally smaller than the shaft and may have stretched or worn during operation.

Measuring procedure

1. Clean the shaft surface.

2. Measure the shaft with a caliper or micrometer.

3. Take measurements at three or more positions.

4. Check the sealing area for grooves, scratches, or uneven wear.

For example, if the shaft measures approximately 35 mm, the required oil seal will normally have an ID of 35 mm.

2. Measure the Housing Bore — OD

The outside diameter corresponds to the bore in which the oil seal is installed. Measuring the housing bore is usually more reliable than measuring an old seal that may be damaged or distorted.

Measuring procedure

1. Remove the old seal carefully.

2. Clean the housing bore.

3. Measure the bore with the inside jaws of a caliper or a bore gauge.

4. Measure in several directions to confirm that the bore is round and undamaged.

For example, if the bore measures approximately 52 mm, the required seal will normally have an OD of 52 mm.

3. Measure the Seal Width

Use a caliper to measure the seal from the front face to the rear face. Measure an area that is not bent, crushed, or damaged.

If the seal has a flange, shoulder, or special profile, record both the main width and the overall width.

Oil seals with the same ID and OD may have different widths, such as:

• 35 × 52 × 7 mm

• 35 × 52 × 8 mm

• 35 × 52 × 10 mm

The width must therefore be checked before ordering.

Check the Seal Design

Oil seals of the same size may have different designs, including:

• Single-lip seals

• Double-lip seals with a dust lip

• Seals with or without a garter spring

• Flanged or shouldered seals

• Directional or bi-directional seals

In most applications, the spring-loaded side should face the oil, grease, or fluid being retained.

Check the Seal Material

The material must be suitable for the fluid, temperature, shaft speed, pressure, and operating environment.

Common materials include:

• NBR: Suitable for general oil and grease applications

• FKM: Suitable for high temperatures, fuels, oils, and chemicals

• Silicone: Suitable for a wide temperature range

• EPDM: Suitable for water and steam, but generally not petroleum-based oils

• PTFE: Suitable for chemicals, high temperatures, and low-friction applications

• Leather: Used in some industrial and heavy-duty applications

Do not identify the material by colour alone, as colour standards vary between manufacturers.

Check Metric and Inch Dimensions

Oil seals are available in both metric and inch sizes.

Examples:

• 35 × 52 × 7 mm

• 1.375 × 2.000 × 0.375 inch

Measurements such as 25.4 mm, 50.8 mm, or 9.5 mm may indicate an inch-size seal. Do not round an inch dimension to the nearest metric size, as the seal may become too loose or too tight.

Information to Provide When Ordering

To help the supplier identify the correct replacement seal, provide:

• ID × OD × Width

• Clear photographs of the front and back

• Brand and part number

• Seal lip and outer-case design

• Type of oil, grease, fluid, or chemical

• Operating temperature and shaft speed

• Shaft rotation direction

• Machine or equipment type

• Details of the leakage or failure

Important Considerations

Replacing the seal alone may not solve the problem if the shaft is grooved, the housing is loose, the seal is installed at an angle, or the selected material is unsuitable.

Before installation, ensure that the shaft and housing are clean and free from burrs, sharp edges, rust, and damage. Lubricate the sealing lip and install the seal evenly in the correct direction.

Summary

The three main oil seal dimensions are:

Shaft Diameter × Housing Bore × Seal Width

The seal design, material, fluid, temperature, shaft speed, pressure, and condition of the shaft must also be considered.

For assistance, send photographs of the existing seal, the measured dimensions, and the operating details to the specialists at Seal4GO.com.

Measure correctly. Select properly. Reduce leakage with Seal4GO.com.