I love these things. If I can help him, I will never use anything else again. I have used both types of butt joint (same outside diameter) and lap joint (different outside diameter). For splices, I use an optional exhaust sealant such as Permatex 80335 Muffler and Tailpipe Sealer, 3 oz. I'm not sure if this is necessary but I like to do my best to make sure I get it right the first time so I don't have to go back and fix it or do it all over again! After some driving and a few days later I check them again and tighten if necessary (also checking for leaks). These things are easy to use and seal really well, with the added bonus that you never have to take them off, you won't damage your pipes which means more work taking them apart and putting them back together!