How to Measure an Oil Seal
Three numbers — shaft, bore and width — are all it takes to find the right replacement. A basic digital caliper and two minutes is the whole job.
The three numbers
Every metric oil seal is described the same way: ID × OD × W, in millimetres. A seal marked 25 × 47 × 7 fits a 25 mm shaft, sits in a 47 mm housing bore, and is 7 mm wide.
Inner diameter = your shaft
Measure the diameter of the shaft where the sealing lip rides. That number is the seal’s inner diameter.
Outer diameter = your bore
Measure the inside of the housing bore the seal presses into. That number is the seal’s outer diameter.
Width = seal thickness
Measure how thick the old seal is, front to back. If it is worn flat, measure the depth of the seat instead.
Step by step
Get a digital caliper
Any inexpensive digital caliper reads to 0.1 mm, which is plenty. A ruler is too rough for shafts under 50 mm — the difference between a 24.9 mm and a 25.1 mm reading decides whether you order the right seal.
Measure the shaft, twice
Take two readings at 90° to each other on the shaft where the lip runs. If the numbers differ, the shaft is worn oval — use the smaller reading and mention it to us if it is severe.
Measure the housing bore
Clean the bore and measure its inside diameter. This is more reliable than measuring the old seal, which may be scored or compressed from removal.
Measure the width
Measure the old seal front-to-back, or the depth of the seat if the seal is damaged. Standard widths come in whole millimetres.
Round to the standard size
Oil seals come in standard metric sizes. A shaft reading of 24.9 mm is a 25 mm shaft; a bore of 46.9 mm takes a 47 mm OD seal — the rubber lip and press fit take up the fraction.
Measured? Find your seal
Enter your three numbers below — stock and price come straight back.
Find a seal by dimensions
Fill in any of the three dimensions — leave the rest blank. All values in millimetres.
Field tips that save a wrong order
- Measure the shaft, not the old lip. A worn seal’s rubber lip is exactly the part that has worn away — it will read small.
- Work in millimetres. Our catalogue is metric; 1 inch = 25.4 mm if you are converting.
- Note any markings. Letters on the old seal (TC, SC, FKM…) tell you the type and rubber — see our cross-reference guide.
- Photograph before binning. A photo of the old seal plus your shaft diameter lets us confirm your reading — send it on WhatsApp.
Common questions
My measurement falls between two standard sizes. Which do I pick?
Round to the nearest standard size — seals are built with interference designed in. If you are more than 0.3 mm off any listed size, the shaft may be worn or the shaft is an inch size; ask us before ordering.
Do I need the width to be exact?
Close is fine. A seal slightly narrower than the seat works; one slightly wider usually still presses in. The ID and OD must be right — those do the sealing and the holding.
The old seal is destroyed. Can I still find the size?
Yes — measure the shaft and the bore, note the space available for width, and tell us what the machine is. Between the two measurements and the application we can almost always pin it down.
Every standard metric size, searched by your numbers
Once you have your shaft, bore and width, our catalogue search takes the three numbers directly — no part numbers needed. We stock metric rotary shaft seals from 4 mm instrument shafts to 300 mm industrial shafts in NBR, FKM and other compounds, with live stock shown per size. If your measurement matches nothing listed, the size may be worn, imperial or non-standard — send us the numbers and a photo and we will match it by hand.