Here is a summary of the Mugen Header Install on my 2008 Fit... I bought it used from PanDucky MotorSports and did not have any instructions so if I missed anything let me know
Anyway I've installed a lot of headers and have to say that this was the easiest install that I have ever done. Looking over the header it is typical Mugen quality... nice piece of high quality workmanship!
STEP1; Remove the heat shield off of the stock exhaust manifold. Easy to do as it is held on with three bolts that are all easy to reach with a socket wrench with a swivel joint and short extension.
STEP2; Loosen the three bolts and two stud nuts that hold the exhaust manifold to the head. Just loosen for now.....
STEP3; Remove cat or in my case the T1R test pipe (This is optional and would probably not do next time. I just did to have more room to work since I'm working in my driveway and not on a lift at a garage.)
STEP4; Loosen bolt holding the manifold support bracket and remove the bracket to manifold bolt. This is a must in order to remove the stock manifold.
STEP5; Take off the three exhaust manifold bolts and two stud nuts. Unclip the O2 sensor wire and remove header from the head. Place on floor next to the new header and say "Wow this header looks sweet compared to the stock manifold.
STEP6; Remove o2 sensor from stock manifold and put on new header.
STEP7; Place Mugen header on the two studs... put the three bots and two nuts on loose, do not tighten yet. Attach O2 sensor.
STEP8; Attach manifold brace... again do not tighten yet.
STEP9; Attach Test Pipe and tighten everything now... Tighten test pipe bolts and manifold brace bolts.
STEP10; Tighten the three header bolts and two stud nuts.
Done... Look everything over... make sure everything is tight. Take for test drive and check for leaks or weird noises. If everything sounds good then take for seat of pants dyno run to put a big smile on your face
****Remember I did not have any instructions but have installed a lot of headers over the years. So these are the main steps to follow. They should help if you have not done this before. Feel free to pm me with any questions. Always looking to help fellow Honda brothers and sisters out!