O2 sensors will fit in snugly where the factory sensors go. Most header pipes have O2 Bungs in them now. Most companies(except V&H) have placed them in such a place you can get to them with an O2 sensor removal socket or can get to it with a 7/8" wrench. Factory headers, D&D, Rinehart and most the others have figured this out. There is no problem with the seal as all the sensors have a metal crushable gasket on them that will snug right up to the O2 bung. The issue is that sometimes the factory runs them in tight and it may take a little effort to get them out. You can tell if they bind so I sometimes have to lubricate the threads and run them back in and out. If you ever put new headers pipes on make sure to run a tap in the O2 bung as most of them will chrome the pipe after the bungs are installed. The bung threads will have Chromed up as well making the O2 sensor "gaul" on removal. ALWAYS USE ANTI-SEIZE when putting plugs or O2 sensors back in. You also do not need to overtighten them, just snug up plus a little. If a bike has O2 bungs and I can get my sensors in there I always try to tune this way. The copper tubes and vacuum pump method work but also can be problematic with the cooper tube collapsing under the heat. I really have to pay attention to this to make sure readings are consistent. Either method will yield good AFR data you just have to pay attention. One more little thing that can get overlooked is that these wide band O2 sensors used for tuning need to be calibrated in free-air before each bike. Keeps the data consistent.