TylerB
Senior Member
Yes you have to bore the hole out bigger so it fits. No it wont affect anything to do with your air/fuel ratio.
Your fwd two O2 sensors control your Air/Fuel ratio. Your rear two only ensure your catalytic converter is operating properly. They're probably only reading bad, because you put true duals on and they're reading too much hydrocarbons that are not being burned off any longer.
Just a FYI. The 2nd set of 02s mess with the AFR too. If they detect too high of a temperature then they will signal a cat protect mode which adds extra fuel to richen the mixture to cool the cats.