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Spark Plug Indexing a cautionary tale...

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492 views 14 replies 8 participants last post by  VII7  
#1 ·
First a shout out to member Onixxf who brought this subject to my (and our) attention a few weeks ago. Frankly, I had never heard of Spark Plug indexing till our conversation here on the forum and I have been in the car business for 40 years. But even old dogs can learn new tricks.

To be clear this concept applies only to modern direct injected engines, including some R172 models and generally to the high performance versions. Still if you have a direct injected engine and work on it yourself you need to pay attention! Check to see if it applies to you.

The concept is simple, when installing spark plugs after the final torque the spark plugs open side of the gap needs to face the injector +/- about 22 degrees to either side. Very bad things can happen if this is not adhered to, hence the subject of this post.

I religiously watch a YouTube channel called LegitStreet cars. The YouTuber is a former Mercedes Tech named Alex. His following is over 1.5 million and he's an excellent tech. Watching his latest episode last night, to my surprise, the subject of Spark plug indexing came up in a big way! It caused the total destruction of a 2014 Mercedes E63 AMG M157 engine with only ~54k miles that was otherwise perfect with a perfect service history!

If this interests you I urge you to watch the video. You will not be disappointed.
H

 
#3 ·
I'd watched this video last night and will be changing my spark plugs myself on my R172 SLK55 this week together with my friend's R172 SLK350, both of our are direct injection engines. We'll be using a belts and braces approach by marking up the socket wrench so we can gauge by the marking when the open ended electrode faces the injector within the allowed range when tightened to the recommended 23nm. I'll be using genuine Mercedes plugs on my SLK55 whereas my friend will be using genuine Bosch plugs for his SLK350.
Youtube tutorial will be out on my channel in due course .
 
#4 ·
As previously stated, not all direct injected engines require this procedure. That holds true even with the Mercedes family of direct injected engines. The takeaway here is simply that if you're working on one check the factory recommendations before you proceed.
H
 
#9 ·
ccmano said:
As previously stated, not all direct injected engines require this procedure. That holds true even with the Mercedes family of direct injected engines. The takeaway here is simply that if you're working on one check the factory recommendations before you proceed.
H
Click to expand...
And its always a good Idea to work with a Torque Wrench on the most of Bolts , Screws when u work on the Engine. OK Iam Aircraft Engineer maybe
occupational disease.

But good to know, my SLK became new Sparks shortly BEFORE I buy it. After next Oil Change we found out that the Shop who done the Inspection "forget" One O-Ring on the Filter so better check the Torque on the Sparks. THX for sharing this Information.
 
#7 · (Edited)
Here is the Mercedes service bulletin presented in the video. Note that it was issued in 2014. No doubt there are now more engines in the Mercedes family that require indexing.
H
 

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#8 ·
Thank you thank you thank you for raising this issue and reminding me of this. Not sure if it’s necessary on my 55 but I will need to be sure. Again, THANK YOU
 
H
#10 ·
To add to this thread, I offer up my 'indexing quandary': I am happy owner of 2013 SLK55 with M152 and 55K miles, which requires indexed plugs. I order 8x Bosch plugs from FCPeuro (who have been great), and start in on the easiest cylinder I can access (Cylinder 1, front most passenger side). Remove plug, and finger-tight the appropriate replacement that I have marked with red paint: shows as 9 o'clock at finger-tight, then I torque to 23 nM and I'm at roughly 9:30 o'clock (290°)... clearly not in the 45° window semi-spec'd by MB in their 2014 tech bulletin. I try again with 2 other plugs across both Cylinders 1 and 2, but have the same results (at least they are consistent!) I reach out to FCPeuro about the issue, who makes it very easy to try out 4x MB-logo plugs (which FWIW seem to be the same Bosch plugs but with an MB logo on it). I try the MB plug, but ironically it's more off-target than the Bosch plugs are: I'd estimated it around 245°, when 0° is the target.
At this point I have concluded:
  • there is verifiable 'tolerance drift' between the spark plugs of 2026 and those from 2013, or the factory was re-tooled and someone missed the spec, and therefore no plugs I purchase today are going to index correctly into my 2013 M152
  • I could 'grind down' the crush washers on the plugs I have purchased, but that carries risk as my ability to shave off 0.01mm of depth evenly is in doubt
  • I could purchase an indexing washer set (JEGS.com), but those washers are 'loose' while the Bosch are 'captive'. Does it make a real difference? And I have to assume the JEGS aren't German... is there a difference in the crush characteristics?

Bottom Line: It seems I cannot purchase a replacement plug in 2026 that will index correctly into my 2013 engine. Given that, what are the options to correctly seat plugs? 3rd party indexing washers? Manual grinding of the OEM washers? Something else? Blindly trusting the OEM Logo seems like a farce at this point, but I'm open to being wrong on this topic (as It is certainly easier to just trust the MB-logo)
 
#11 · (Edited)
Interestingly, at the end of the video I posted, Alex came to the very same conclusion and showed it. Torquing factory plugs (or any others) to the required spec did not place the electrode in the correct position. His solution was to grind the washers down until the plug indexed properly. This really makes you wonder how many dealers let alone independent shops go to this trouble. His take on the service bulletin was that MB was protecting themselves from liability should engines under warranty have this type of failure. Really makes you wonder what's going on here.
H
 
H
#12 ·
Indeed, I foresee sandpaper in my future (although I have an indexing washer set inbound - perhaps it will save my finger tips).
 
#13 · (Edited)
Per Alex, the washer thickness that worked for him was 1.30 to 1.35mm. I suspect it may vary depending on the thread orientation of the spark plugs and the head. He used this setup to grind the washers down.
H
Machine tool Machine Saw Blade Tool
 
#15 ·
I have changed plugs on a handful of different indexed engines and they all indexed within the acceptable range. Last time was about a year ago with MB plugs, not sure if something could have changed on the plugs and no one at MB would notice.
 
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