How to read and measure an oil seal size
What the bore, outer diameter and width in a size like 40X62X7 mean, how to measure correctly, NBR vs FKM, and what to check.
Last updated: 6 November 2026
An oil seal size is written with three numbers: 40X62X7 means a 40 mm bore, a 62 mm outer diameter and a 7 mm width.
| Measurement | What it matches | Symbol |
|---|---|---|
| Bore | The shaft diameter | d |
| Outer diameter | The housing bore | D |
| Width | The thickness of the seal | b |
Measuring accurately
An old seal may be crushed and its lip worn. So where possible, measure the seat rather than the seal:
- Measure the shaft diameter with a caliper on a clean, unworn section.
- Measure the inside diameter of the housing.
- Take the width from the old seal or from the depth of the housing.
A decimal value (such as 101.6) may mean an inch seal.
Material
| Material | Short name | Approximate temperature | Use |
|---|---|---|---|
| Nitrile rubber | NBR | −40 to +100 °C | General purpose, oil resistant |
| Fluoroelastomer | FKM (Viton) | −20 to +200 °C | Hot and chemical environments |
NBR or FKM next to the size gives the material, as in 40X72X10 NBR.
Lip type
- Single lip: keeps the oil in, suits clean environments.
- Double lip (dust lip): the second lip keeps out dust, suits dusty environments.
The letter code for the lip type varies by brand, and different types exist in the same size.
When fitting
- The oil-retaining lip should face the oil side.
- Lubricate the lip before fitting.
- If the shaft surface is scratched or worn, a new seal will leak too.
Enter the size in the Belt & seal finder to see options and stock. Example: the 40 × 62 × 7 page.