i haven't done a timing belt on an xterra, but i have done it on my 92 pathfinder two times. the Haynes book i used the first time said to remove the valve covers but it is unnessary and i did not do it. i would change out both cam seals and the front crank seal while i had it apart, and also new belts and hoses. one more thing, after you get it back together you may want to check your timing. this past summer when i did the pathfinder the timing was way off when i got done,guess the old belt was stretched. it is a pretty easy job if you take your time.