4. The pipe of choice for this hop-up was the ThunderHeader 2-into-1 long pipe in a matte-black finish with matching heat shields. Since we are installing a closed loop system we needed to weld a set of O2 sensor bungs on to the pipes. Eric drilled the holes into the header pipe around the same area as a stock H-D application. Then he welded a set of bungs into the header pipe. Once the pipes cooled down he cleaned the threads with a tap.