O2 Sensor Bung Hole Plug

[showerfan]

Anyone know what size bolt will plug the hole in my S4RS exhaust header — on the left side of the bike, just above the foot peg? I assume an O2 sensor used to be there, but the bike has been tuned and I’m not sure what was there more recently. I only know it didn’t used to be an open bolt hole, and now it shoots blowtorch-blue flames out at my foot when I ride, which hurts after a while. Also it backfires like crazy and goes through fuel like a demon.