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C400: Changed my Spark plugs today... Some thoughts
Like the title says, I changed the spark plugs on Hans today. Overall definitely a fairly easy job if you're handy, but I can see where novices, and people who just don't do their own maintenance would get overwhelmed.
I took a fair bit of video of the various "stages" of the repair, which I plan to put on YT later this weekend, if anybody's interested.
A couple of random thoughts. While doing my research prior to starting this job, I ran across a fair few threads talking about "Spark Plug Indexing." I had never heard of this before, and I was a little worried that it might be a complicating factor of what is, otherwise, a pretty straightforward operation.
After a bit of thought, I decided to ignore it, and simply ensure the plugs were properly threaded and torque'd. Evidently I chose correctly because the car started right up, purred nicely and pulled hard on a shakedown drive.
Last thing, I used anti-seize. I know NGK and MB don't endorse that practice, but I've been maintaining my own cars for 25 years, and I tend to stick with proven practices. So a VERY modest amount of copper based anti-seize was applied 3-4 threads away from the electrodes.
Oh, and here's the old plugs. Driver side bank, far left plug is cylinder closest to front bumper. Passenger side bank, far right plug is cylinder closest to front bumper.
Wow, your plugs look great! How many miles on those? They look almost new.
The proper indexing is accomplished by a combination of the crush washer on the plug and applying the correct torque. That's it, that will set the electrode in the correct position.
You've used anti-seize on a steel fastener/plug in an aluminum block before? Just curious, I've never really dabbled into using anti-seize before... even though there have been times I wished I did!
Wow, your plugs look great! How many miles on those? They look almost new.
The proper indexing is accomplished by a combination of the crush washer on the plug and applying the correct torque. That's it, that will set the electrode in the correct position.
You've used anti-seize on a steel fastener/plug in an aluminum block before? Just curious, I've never really dabbled into using anti-seize before... even though there have been times I wished I did!
Yes, the plugs do look pretty fantastic, I must say. The gap on the old ones was so close to the new ones that you could really only tell they were used by the discoloration. The tip of center electrode on most of the plugs was "bluged" out a bit compared to the new ones, but overall they were in good shape, and could have been left in place for a few thousand more miles. These have 44K on them.
[EDIT] it's not so much the Block material you have to worry about as it is the Head's material. Every motor I've owned since 1998 has had an aluminum cylinder head (even the 1982 Volvo 242 Turbo), and anti seize has never been an issue.
I have used anti-seize on every spark plug on every engine I've ever serviced. From a lowly 2.0L Ford Zetec, to the Audi 4.2L V8. I've had ZERO issues.
Last edited by TwoC400s; Apr 23, 2022 at 05:35 PM.
Nice video. The 3.0 M276 TT has far better access to both banks, in particular the left/driver's side. The 3.5 M276 NA is much more difficult on the driver's side, requiring either intake removal, or removal of the ECU and fighting vacuum lines.
I advise against anti seize in a dry torque spec application. Not trying to persuade you otherwise, just indicating that I would avoid it.
I agree with Chassis here.
The index is determined by a dry application tq spec. Adding anti-sieze will allow for additional plug turn past the ideal index location. This would in theory make for slightly less efficient burn, but i doubt you would notice.
Also stock interval is like 55k, but if you are going to push more boost and mods, most tuners recommend more frequent changes. There really isnt enough time or miles to have the plugs get oxidized and stuck in the block.
Ive done them on my C400 and the wifes ML (3.5L M276) twice. The 3.0 is a piece of cake, especially if you have aftermarket intake.
I agree with Chassis here.
The index is determined by a dry application tq spec. Adding anti-sieze will allow for additional plug turn past the ideal index location. This would in theory make for slightly less efficient burn, but i doubt you would notice.
Also stock interval is like 55k, but if you are going to push more boost and mods, most tuners recommend more frequent changes. There really isnt enough time or miles to have the plugs get oxidized and stuck in the block.
Ive done them on my C400 and the wifes ML (3.5L M276) twice. The 3.0 is a piece of cake, especially if you have aftermarket intake.
Thanks for laying out all the reason and why I only torque'd to 20.9nm. Wet torque isn't a big mystery, pretty common for wet torque specs to be about 10% less than dry. Which should take care of the indexing as well.
Unless you're using a torque wrench that is lab grade, +/-1.5nm is going to be inside your margin of error anyway.
My results so far seem to have borne out that we're obsessing over something that makes very little difference.
The reason for indexing is not fuel economy or emissions, but rather to avoid spark erosion on the edge of a valve, causing valve failure. The MB document on this is posted dozens of times on this site.
I agree with Chassis here.
The index is determined by a dry application tq spec. Adding anti-sieze will allow for additional plug turn past the ideal index location. This would in theory make for slightly less efficient burn, but i doubt you would notice.
Also stock interval is like 55k, but if you are going to push more boost and mods, most tuners recommend more frequent changes. There really isnt enough time or miles to have the plugs get oxidized and stuck in the block.
Ive done them on my C400 and the wifes ML (3.5L M276) twice. The 3.0 is a piece of cake, especially if you have aftermarket intake.
I'm right at 55K miles stock. Can one go over like 3,000 miles like the naturally aspirated or is bi-turbo more sensitive?
Last edited by Nicholi2007; Jan 14, 2026 at 06:29 PM.
I'm right at 55K miles stock. Can one go over like 3,000 miles like the naturally aspirated or is bi-turbo more sensitive?
Neither you nor the car will notice an additional 3,000 miles assuming there are no other major issues. If your car was experiencing misfires, excessive oil/fuel consumption, overheating, or power loss (generally any other issue that could contribute to spark plug wear) I would tell you to change them early.
However, a few thousand miles on a car with no issues is no problem.
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