All 8 cylinders experience the same wear and tear. When problems develop with one, you should suspect that the others will soon follow with likely the same malfunction.

 

In my above post #20, I noted that I acquired a set of 8 MB p/n 0001590500 OE spark plugs that also carried an NGK p/n SILZKFR8E7S marking. I wanted to examine the plugs to determine if they are manufactured identically to self-index when installed and torqued to spec. If they are identical, each plug’s ground electrode should have the same radial orientation when seated.

Using a 20mm thick piece of PVC plastic, I drilled and tapped a hole with an M12 x 1.25 thread to accommodate this spark plug whose threaded shank is about 27mm in length. Screwing in the first spark plug, I seated it finger tight and marked the location of the ground electrode’s weld point.

Then, one by one, I screwed each of the remaining 7 spark plugs, noting the location of each plug’s ground electrode weld point with the plug seated. All 7 spark plug electrode welds were within just a few degrees of the reference mark of the first spark plug.

From this, I conclude that this MB/NGK spark plug will index properly assuming the engine’s spark plug receptacles are machined for indexing. Being identical, any plug can be installed in any bank location.

There is an interesting YT video (about 5 minutes long) from the Science Channel program “How It’s Made” that shows spark plug manufacturing at Bosch ( ). About halfway through the video, the spark plug’s ground electrode is attached as a straight metal piece, and the spark plug metal base is inserted into a collet. The collet spins while a synchronized cutter carves the threads into the spark plug shank – all within about 1 second. Nothing is said about indexing, but this automation demonstrates how this can be easily accomplished such that each spark plug is identical.

When I install these plugs, I’ll mark their tops and check to see if all 8 plugs seat in identical radial orientations in their respective locations.

 

Just want to add I changed the coil for cylinder 4 and it fixed the problem. Seems to me it's just about time for them to fail all within 10,000km of each other. My suggestion would be to change them all. There are only 4. Save you going back and forth but it's a very easy job for the mechanic.

 

Be careful so none of that loose stuff doesnt fall into the cylinder. I’d put a strong vacuum cleaner into those areas and suck out what you can.

Theres mixed feelings on using anti seize on the plugs. I use just a little and it eliminates that.

 

100% agree with blowing out and vacuuming out any debris that is in the spark plug well.

100% Disagree with using any anti-sieze. The spark plug's ground is the threads themselves you do not want to put anything there that reduces metal to metal contact. All modern spark plugs have a coating on the threads to prevent seizing in dissimilar metals. Anti-sieize is also a lubricant that alters torque values. Over tightening a spark plug because the threads are lubricated with anti-sieze can not only break the plug, but it would also change the indexing, which was the focus of this thread.

Here's a Mercedes ​​​​​article on spark plugs. Use of anti-sieze is discussed on the last page:
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Here's NGK's tech bulletin stating it is not necessary to use anti-sieze.

 

I made every effort to index new plugs on the first plug change for my 2018 e400 (50,000 km). However, the as-found condition of all 6 factory plugs had a breakaway torque of ~22Nm and where all indexed to the 9 o'clock position. All new MB plugs were installed at ~22Nm and as-left at the same 9 o'clock position. This is about 90 degrees out from the fuel injectors and contrary to MB saying they should be aligned toward the injectors. Just sayin...

 

I changed the plugs on my 18 S63 a couple months ago. I marked my socket so I could track the electrode position. Every plug indexed properly with the exact same orientation, literally within 5 degrees. I was amazed at how well that worked!