Spark Plug Gap
 

2005 C55. I've seen in listed as 0.031 in and 0.040 in. Which is it? OEM plugs.

Was this that stupid of a question?

I think Bosch plugs are pre gapped at .40
ngk is .41 I think

So the owners manual I found says they are gapped at the 0.031. The new ones I looked at are all listed at 0.040. So you don't change the gap to what the owners manual says, just leave them as them come?

Just leave them as they come.

If misses like crazy at WOT you may have to close the gap a bit

The correct plug for C55 is NGK ILFR 6 A, gapped to .031 in, torqued to 18-22 ft/lb.

The NGK plug is preset at .031 in. If a different equivalent plug is used and not preset to .031 - re-gap it.