From the other threads together with Bosch site, I found 4 choices of plugs for my 2004 CLK320.
Platinum Plus (OE, single contact); Platinum + 2; Platinum + 4; Platinum IR Fusion

MB dealership uses platinum Plus for my car with part number = 003-159-66-03-26 and it costs $10.90 CAN each.

I checked the net and found that I can get them a lot cheaper. Since I am not getting them from the dealership, I am considering to get other Bosch plugs. Will the other plugs (+2, +4, IR Fusion) benefit my stock 3.2L engine? How about durability; are they all the same?

 

I would get (and have gotten) Bosch FR8DPP33's. If I went with another plug, it would not be a 'plus' plug. Here's why -

Having indexed plugs, I recognize the folly in +2 and +4 plugs.

Even if I had no intention of indexing a plug, electricity follows the path of least resistance. You could have thirty ground electrodes and the spark will jump to the 'single' ground electrode with the least resistance - ignoring the others.

You have a dual plug head. There was a time when machine shops were paid big money to convert single plug Porsche heads to dual plug heads. I don't know of anyone who has ever paid for dual plug heads and then used 'plus' plugs. There's a good reason for it.

 

Is FR8DPP33 the stock spark plug? I thought Platinum Plus (7422) is the stock plug.

I did a search in Bosch site for 2004 CLK320 and it resulted in the following 4 parts: Platinum Plus (OE, single contact); Platinum + 2; Platinum + 4; Platinum IR Fusion.

 

Every Bosch plug has a stocking number and a part number. 7422 is the Bosch stock number for part number FR8DPP33. They are the same plug.

Take a look here at the fourth plug in the premium class. OE is an 7422/FR8Dpp33

 

OK. I understand.

What threw me off was you said "If I went with another plug, it would not be a 'plus' plug."

Looks to me that the OE plugs are Platinum Plus with code 7422/FR8Dpp33.

Thanks. I will just get these OE plugs. Too expensive to buy from the dealership, I will look for another shop with more reasonable price

 

How can I remove the spark plugs? They seem to have a metal cover over them. do I just pull off the cover, or is there a twist/unlock required?

How much torque do I use to put them back in? I heard 11-14 ft-lbs for some other car.

 

Sorry, it would have been clearer for me to have written plus2/plus4.

 

My mileage are mostly highway, 5-hr run back and forth between Montreal and Toronto every few weeks. Can I assume my plugs are nice and clean?

How often should the plugs be changed, every 100K KM?
The specs for these 7422 plugs calls for GAP=0.040. Is this gap size good for my CLK?
I can see that the price varies a lot in the net. Amazon.com sells them for $4.39 each while Rockauto sells for $7.51 ($10.90 CAN in the dealership). Where is a reliable place to get these plugs?

 

The plugs should be changed every 100,000 miles / 5 years, whichever comes first. The original gap was 0.040”

Now for the *touchy* part: They should be tightened to 28 NM (20.5 ft/lbs), but everyone handles this a little differently. I use anti-seize on anything that sees aluminum threads. Some are aghast and say I'll foul the plugs. I won't say it's impossible, just that it's never happened to me. Also, there's the issue of wet and dry torque (reduce it 40%, or 60%?), and whether it's safe to use a 10-year old, never been calibrated torque wrench.

A long time ago Bosch said 90 degrees for flat seats, and 15 for conical. That's what I do. No torque wrench required.

 

For my CLK320, I will need 12 of these 7422 plugs.

I am thinking not to get them from the dealership. Not sure if I want to buy them from Amazon.com ($4.39 each). How is Rockauto? Any other relibale mailorder shops I should consider?

 

When I changed my spark plugs on my car and later on my parent's ML, I got them from Amazon.com from the Parts Authority vendor. The shipping was fast and got them within 2-3 days from when I ordered.