I'm stumped; I removed the 6 long bolts holding the cover over the coils on my 94 E320 to change the spark plugs today. I have done this several times during my ownership of this fine car. After I changed all of the plugs, I put the cover on and insert the 6 bolts. I can tighten just 4, but 2 jsut spin, as if the threads are stripped. So I remover the cover, and put the 2 bolts into the threaded holes and gently turn, and the threads catch. But in serveral more tries with the cover in palce the bolts don't engage the threads. I can't find any interference that would keep the bolts up and away from the treads in the hole.
Now the car runs fine with new spark plugs. but I have 2 bolts just sitting in the holes without any engagement of the threads.
Any ideas?
regards,
Mark

Are there different length bolts ???????????

Stripped threads.
There are three answers:
#1. HELICOIL® the holes. http://www.helicoil.com/
#2. Re-tap the holes for a slightly larger size.
#3. Replace the valve cover.



Have a great day.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

Is the coil cover flat with the valve cover all the way around????????
If not , check the plug wire routing..may be lifting the cover just enough

your question gets me thinking of axing that cover completely. I think the air intake pipe is screwed on to it but the arrangement is not stuck in my head. I experienced the same thing on my 97 E320. I already axed that other cover over the fuel injectors and I am this close to getting rid of the front cover with the star after it dislodged one day and was getting chewed by the fan. Anyone have any objections to getting rid of the coil cover? Coils are not being cooked and maintenance is much easier. (By the way was your cylinder #6 plug oily?)

Remove all the bolts and compare their lengths...is it possible you may have them switched?

__________________
...Bob

1994 E320
2001 Corvette Coupe
1998 Jetta
1996 Camry

The bolts are all the same length. I don't think the holes are stripped, because If I remove the cover, I can thread the bolts into the holes in the head. I've checked the routing of the wires thinking they could be interfereing with the position of the and don't think this is the problem, but will look at this again after work today.
No, #6 plug was not oliy; all were in great condition after one year of service.
Mark

My coil cover warped slightly at one time, causing some of the bolts to not engage properly. I lived with that for awhile until I did some tinkering with plugs and coils later on. Forcing it only served to crack the cover in half!

Car runs fine regardless, but BEWARE of introducing lots of water to the motor exterior if your cover doesn't have a good seal (for those of you who do wash their engines)...

...has anyone ever found a source that supplies replacement covers?

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

I did not consider warping, and will check for this.
regards,
Mark

You may be on to something. One of the great joys of owning an aging Mercedes is throwing away superfluous parts, of which there are many. The cover keeps the coils dry and reduces debris in the spark plug holes but those seem like manageable issues. You should blow out the spark holes with compressed air with cover or no cover. If it was me I'd go for it - probably keep the cover for when the car is sold.

I've used the Time-Sert Thread Repair Kit many times. I keep several sizes in my shop. Similar to Heli-Coil, quick, easy, and effective.

water is not an issue for me. I dont wash engines, I just wipe them off. I dont know why anyone would ever do it on modern engines.

Still stumped; can't get 2 of the 6 bolts to catch on the threads. Removed the cover today; looked for any interference, and found none. Checked the cover for warpage with a straight edge across the three towers in the long direction, and it is flat. Even stood on the cover (180 lbs) as I tried to tighten the bolts with no effect. I'm done, but 4 out of 6 ain't bad.
Mark

Must be just the very top of the threads are stripped. if there are washers under the screws . try removing them or get a slightly longer screw.......
You can usually see aluminum on the screw ends if the holes are stripped

The washers are captive; won't pass the threads of bolts. I will try to find a difference in the end of each bolt's thread tomorrow looking for a stripped thread at the end of each.
regards,
Mark