what is important also...you should not do this job ..if you dont know what you are doing.of course its not a very difficult job...but things can go wrong.and if you can afford it...id advise you to let someone who knows these things ..to do the job for you.
you should also change the cambelt when you do this job.its been said that a cambelt should not be tighten and run..and loosen and then retighten ..and its always good to know that the cambelt is in perfect shape.