2. First on the to-do list was the wide band O2 sensor. We drilled a 7/8-inch hole with a hole saw and a 90-degree drill 3 inches back from the header’s collector. We chose the location forward of any welded joints to avoid any possible leaks that might contaminate the O2 sensor’s readings. Also, the hole is drilled a little above the centerline of the pipe to put the sensor at an angle of 10 degrees for condensation runoff.