Indexing Spark Plugs

Somewhat off topics, but an interesting read and interesting info for those who care.

I needed to change the spark plugs in the wife's Mercedes GLC300 SUV which has a 2.0 L turbo Direct Injected engine. Researching on the web to find the right spark plugs I stumbled across a Service Advisory instructing Service Technicians that the spark plugs needed to be indexed . The reasoning is that if the back side of the ground electrode is pointing towards the injector, the ground electrode can / will deflect the spray pattern of the fuel and can cause washdown of the cylinder walls and / or high density fueling at the edge of the piston crown causing errosion of the piston crown.

I also came across websites suggesting that, if you buy new sparkplugs from Mercedes, they come preindexed. HMMMM.
Also came across posts saying that the neither bought Mercedes spark plugs nor did they index the aftermarket plugs and of course, they had no problems. So who to believe?

The only aftermarket plugs for this vehicle are NGK EVO's and the stamping number on the base of the plug is identical to the spark plugs that come from Mercedes. Interesting.

The tolerance for indexing allows the spark plug open end of the ground electrode to be off center to the injector +- 45 degrees,
The spark plug thread pitch is 1.25mm
The spark plug seal ring is 1.5mm thick

So I buy aftermarket plugs (4) and saved $50 CDN.from what I would have had to pay at Mercedes.
Thinking that I may have to index the plugs, I bought a bag of 30 copper sealing rings 1mm thick from Amazozn for $13 CDN.

Time to get into it. I marked the first sparkplug #1 cylinder and removed and found the ground electrode pointed exactly 180 degrees out, meaning the back side of the ground electrode was pointing directly at the injector. I remember that when I bought those plugs, I had to get them from Mercedes since there were none available aftermarket, ( 4 years ago) so according to the posts I had read, they should have been preindexed but obvoiusly not.
Go and test install a new aftermarket plug and sure enough, it's out exactly 180 degrees. HMMM. Tried test installing the 3 other new spark plugs and all of them pointed out exactly 180 degrees so that didn't help.

On to the plan and change out the seal washer from 1.5mm stock to the 1mm  washers from Amazon. According to my calculations, that should increase the thread depth by 40% of 1 thread pitch = 144 degrees. Installed the spark plug and it fell within 20 degrees of the centerline of the injector. That's well within spec.

Continued on with the same process and found the other remaing original spark plugs clocked out exactly 180 degrees of of spec. Removed the 1.5mm seal rings from the 3 remaining new plugs and installed the 1mm seal rings and all 3 new plugs clocked out exactly the same as the 1 plug. Job done 2 hours later...

Why index spark plugs?? Racers have been doing it for years. Pointing the open end of the sparkplug at the intake valve has shown to increase horsepower by 3-5 %. I don't completely understand the principal but believe that by doing so, introduces the spark in the chamber in such a way that is more effective an able to light off the turbulent fuel.

Things you learn when you least expect it.
Hope some of you find it interesting. Who'd a thunk that changing spark plugs is not as simple as it used to be. Especially if you happen to have a direct injected engine. Now I have to check my Ford Maverick which also has a direct injected engine. More work !@#!@#$
KB