What Is a Rotary Shaft Seal, and How to Replace It

What could be more annoying than leaving oil streaks in your car’s parking space? These seepages are often the result of defective engine seals. But what exactly are they? We tell you everything in this article!

Seal: its characteristics

The rotary shaft seal is a seal that allows the sealing of a rotating part.


The seal is a circular seal made of elastomeric rubber.

It consists of the following elements:

    – An elastomer body whose outside diameter is pressed into a fixed part (housing, pump body, etc.).

    – A reinforcement which reinforces this body.

    – A sealing lip that rests on a rotating cylindrical part (shaft, crankshaft, etc.).

    – A spring in the form of an O-ring housed in the inner part of the lip, whose role is to create pressure on the lip to increase the seal.

    – An external anti-dust lip (optional). If present, this is a double-lip seal.

Types of seals

Seals are determined by their dimensions:

    – Ø inside: this is the diameter of the shaft.

    – Outside Ø: this is the outside diameter of the seal and the bore (circular hole in the body in which the seal is placed).

    – Thickness: it is the distance between the back and the front of the seal.

Other codes are added to identify the seal and its specific characteristics that determine its use.

For example, let’s analyze a complete SPI gasket reference and its different variations: 14 x 43 x 10 – MEWLR – NBR.

    – 14 represents the inside diameter.

    – 43 is the outside diameter.

    – 10 is the thickness.

    – MEWLR is the type of seal with the following characteristics:

        ◦ reinforcement (embedded, exposed, exposed reinforced or semi-coated);

        ◦ the spring (free, embedded, or exposed);

        ◦ the coating with or without grooves (ridges on the outside diameter of the rotary shaft joint for better hold);

        ◦ the dust lip (with or without);

        ◦ the ridges (oblique ridges on the inner lip which, depending on the direction of rotation, allow oil to be directed inwards).

    – NBR for the seal material, knowing that there are 2 types of elastomer:

        ◦ NBR for nitrile;

        ◦ FKM for fluorocarbon.

In this particular case, MEWLR – NBR means nitrile rotary shaft seal with semi-coated armature and corrugations, dust lip with visible spring.

Replacing the rotary shaft seal

This is a delicate operation due to the fragility of the seal and the need to position it correctly.


It is essential to lubricate the gasket: it must be oiled on its outside diameter to prevent it from being torn off during installation, but also on its inside part (at the level of the lips), to avoid its deterioration at the first start-up, while the engine or gearbox oil lubricates it. Avoid damaging the lips during assembly, as well as the seal coating. Position the seal correctly (perpendicularity and centering).

Note: Often, the location of the lip of the used seal has made a groove in the shaft. In this case, replacing it with a thinner seal is preferable not to position the lip in the same place (risk of leakage).


We have just seen that the replacement seal must be carefully chosen. There are many sources of supply (internet, spare parts stores…), but 2 options are to be retained:

    The manufacturer’s network (agents, dealerships), where you will get an identical seal with the identification of the vehicle.

    – Industrial stores specialized in bearings and seals: few, but they are the only ones who master this knowledge and will be able to advise you on the rotary shaft seal best suited to your needs.

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