Oil Seal Leaking? The Real Causes, and Why the New One Leaks Too

Most oil seal leaks I get called about are not seal faults. The seal is the part that gave up, not the part that caused it.

Here are the real causes, in the order I find them.

1. The shaft has a wear groove

The previous seal cut a track into the shaft. Fit a new seal in the same axial position and the lip sits in that groove with nothing to press against.

Fix it by fitting the replacement 2 to 3 mm deeper or shallower along the shaft so the lip runs on fresh metal, or fit a thin shaft repair sleeve. Do not machine the shaft down and hope.

2. The shaft finish is wrong

Aim for 0.2 to 0.8 micrometres Ra, plunge ground. Too rough and the lip abrades away. Too smooth and the lubricating film cannot form, so the lip runs dry and hardens.

3. Machining lead

If the shaft was turned rather than plunge ground, the surface carries a fine spiral. That spiral acts like a screw pump and lifts oil out from under the lip regardless of how good the seal is. It is invisible without a lead check, and it is a frequent cause of the leak nobody can explain.

4. Fitting damage

The lip is a fraction of a millimetre of contact edge. It does not survive being dragged over a keyway, a spline or a circlip groove, and it does not survive being tapped on from the lip side.

Lubricate it, use a fitting sleeve or tape over sharp edges, press on the outer case with a flat drift, and seat it square to the correct depth. Check the garter spring is still in place afterwards.

5. Pressure in the housing

A standard rotary shaft seal is an atmospheric device. Above roughly 0.5 bar it will start to lift. A blocked breather on a gearbox or crankcase will exceed that quietly and continuously, and you will change seals forever until someone cleans the breather. If the pressure is genuine and by design, you need a pressure rated profile.

6. Wrong material for the temperature or fluid

Nitrile in hot synthetic oil hardens, loses lip load and leaks within months. Judge the temperature at the lip, where friction adds heat, rather than the temperature of the oil in the sump.

• Nitrile NBR, minus 30 to 100 C, mineral oils, general purpose.
• HNBR, minus 30 to 150 C, higher temperature and better wear resistance.
• FKM, minus 20 to 200 C, hot oils, fuels, many chemicals.
• PTFE, minus 50 to 200 C and above, dry running, high speed, aggressive chemistry.
• Silicone, minus 50 to 180 C, dry heat, low speed, food contact grades.

7. Contamination from the outside

Dust, washdown water or grit reaching the lip will cut it. If the environment is dirty, specify a double lip with a dust excluder rather than replacing a single lip twice a year.

How to order the right replacement

An oil seal is specified as shaft diameter x bore diameter x width, for example 35 x 52 x 7. Measure the shaft with a micrometer and the housing bore directly, not the old seal, because a compressed outer case reads undersize. If there is a part number stamped on the case, a photograph is enough for me to cross reference it.

We supply oil seals and rotary shaft seals across the UK with same day despatch on common sizes ordered before 2pm. For pressurised process duties, see mechanical seals instead, and for the naming confusion read lip seal or oil seal.

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Andrew Sykes MCGI, 36 Years Solving Seal Problems

Send the sizes and the fluid to Acumen Seals and I will match it, or tell you why the last one failed.

Questions and answers

What causes an oil seal to leak?

The most common causes are a wear groove worn into the shaft by the previous seal, an incorrect shaft surface finish, machining lead from a turned rather than plunge ground shaft, damage to the lip during fitting, excessive housing pressure from a blocked breather, and the wrong elastomer for the oil temperature.

Why does my new oil seal still leak?

Usually because the shaft was not corrected. A new lip sitting in the old wear track cannot seal. Fit the replacement a few millimetres deeper along the shaft, or use a shaft repair sleeve, and confirm the surface finish is 0.2 to 0.8 micrometres Ra with no spiral lead.

Can a blocked breather cause an oil seal to leak?

Yes, and it is one of the most missed causes. A standard rotary shaft seal is designed for near atmospheric conditions. A blocked breather lets crankcase or gearbox pressure build until the oil is pushed straight past the lip.

What shaft finish does an oil seal need?

Aim for 0.2 to 0.8 micrometres Ra, plunge ground, hardness of at least 45 HRC on hard running duties, and no machining lead. Too rough abrades the lip. Too smooth stops the lubricating film forming. Both leak.

How do I stop an oil seal leaking after fitting?

Lubricate the lip before assembly, protect it past keyways and splines with a fitting sleeve or tape, press on the outer case squarely with a proper drift rather than the lip side, seat it to the correct depth, and confirm the garter spring has not been dislodged.

How long should an oil seal last?

In clean lubricated service at moderate speed and temperature, several years is normal. Life falls quickly with high surface speed, high temperature, contamination and any shaft eccentricity, so a seal that fails within months is telling you about the shaft or the conditions, not the seal.

Which oil seal material should I use?

Nitrile for mineral oils up to about 100 C, HNBR to about 150 C, FKM for hot oils, fuels and many chemicals to about 200 C, PTFE for dry running, high speed or aggressive chemistry, and silicone for dry heat and food contact grades. Judge the temperature at the lip, not in the sump.