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An Article About Replacing The Original Rear Rope Type Crankshaft Oil Seal With A Split Neoprene Seal

Rear Crankshaft Oil Seal


The Mercedes rope seal and a modern alternative

The rear crankshaft oil seal on these engines is a two piece impregnated Mercedes rope seal. Some of the seals were replaced with a thinner impregnated rope and separate rubber backing. Unfortunately in common with many two piece rope seals they often drip oil after a short while. Since around 1980 asbestos is no longer allowed to be used in the seals, this does not help the wearing properties of seals since then.

As technology moved on in later models of Mercedes this two piece seal was replaced with a neoprene one piece circular seal, as was common with other engine manufacturers. This modern type of seal is often used elsewhere such as oil seals for axles, power steering boxes and pumps etc. I have replaced the two piece rope oil seals before on V8 engines with no problem. The Mercedes 300 SEL 6.3 is a far more difficult engine to remove and likewise work on. Having spoke to other people who have replaced these seals they have encountered a small drip within a short while after replacing it.

To remove these engines and fit a new oil seal is a major job and I certainly would not advise anyone to attempt this without suitable prior mechanical knowledge and facilities. It is often easier to remove the engine and gearbox together.

The fact these rope seals often still leak shortly after fitting prompted me to look into designing and having a split seal made. After some investigation the idea I had to make one was already being used mainly in American. Some American companies also produce a solid rubber type two part seal to replace the rope, this is not designed for the Mercedes engine, but this design used in other engines has been very successful and is far superior than the rope seal.

The split oil seal

In short it is basically the same design as the modern type one piece seal except it is a split oil seal to allow it to pass over the crankshaft. The garter spring is also split with a connection hook to re join it together.

It is important that the journal area on the crankshaft where the seal sits should be perfectly smooth and not groved through wear. The Mercedes crankshaft has serrations in it to help deflect oil, however when fitting a neoprene seal it is best to have a smooth surface. To do this you need to have the journal where the seal sits to be 'metal sprayed' at an engineering works and turned back down to the correct 75mm. This is basically having new metal fused on to the journal and then dressed down to the correct size of 75mm. I would not advise this for the main/big end bearing journals but only for an oil seal journal.

Please see the seal below and the fitting instructions I wrote to go with them. If anybody is interested I have these seals for sale. Please see below the fitting instructions which I wrote.

Please click images to enlarge

End view of oil seal with garter spring separate and unconnected.

End on view of oil seal with garter spring separate and unconnected.

Rear view of oil seal with garter spring separate and connected

Rear view of oil seal with garter spring separate and connected.

Front view of oil seal with garter spring separate and unconnected

Front view of oil seal with garter spring separate and unconnected.

The Mercedes 300 SEL 6.3

Guidance notes for the replacement of the rear crankshaft rope seal for the split oil seal on a Mercedes 300 SEL 6.3 (M100) engine.

  1. When fitting the seal ensure the split is facing upwards towards the top of the engine. That is to say that the split should be approximately in the middle of the engine block recess.

  2. 1,It will be necessary to drill six small holes in the sides of the seal casing where it comes into contact with the outer edges of the top bearing case and engine block recess. This is to allow the seal to sit correctly in position over the six prongs. There are three prongs in the top bearing case and lower engine block. The holes may also penetrate the outer section of the rubber seal where it sits against the seal casing. Please note only the outer section can be penetrated!. Before drilling the holes make sure that the hole near the split in the seal is slightly to the left or right and not in the middle of the join. Ensure that the holes are drilled accurately in the seal casing. The six holes should be drilled fractionally larger to allow enough play for the seal to go together tightly and the garter spring to go into position.

  3. 2,Once you have drilled out the holes make sure they all line up by fitting the seal in position with the garter spring and replace the top bearing cap. If everything lines up remove the top bearing case and proceed to number 3.

  4. 3,When fitting the seal slightly roughen the outer edges of the seal and ensure that the top bearing case and engine block recess are cleaned with thinners then generously coat with black automotive silicone sealing compound. This is to stop any possible leak of oil between these surfaces and to bed the seal in position.

  5. 4,Lightly lubricate the seal with engine oil before placing over the crankshaft. The garter spring side of the seal should be facing in towards the engine block. Ensure that the garter spring is fully together before locating on the seal. With the seal on the crankshaft and garter spring still fitted gently lower the crankshaft in position and the seal onto the silicone in the engine block recess, wiping away any excess. Ensuring that the holes in the seal line up with the prongs in the engine block recess.

  6. 5,Apply black automotive silicone in the top bearing cap and bolt this down, again ensuring that the seal goes over the prongs. Use a little jointing compound on the two mating bearing surfaces, as well as on the rubber side slides in the bearing cap.

  7. 6,When bolting the top bearing case down make sure any excess silicone is cleaned away whilst still wet.