I agree with doing belts every 100000k, but timing chains on K motors don't generally give trouble. If it's giving a problem (noisy), no worries, whip it out and change it, but changing it for change's sake is not worth it. Unlike a belt, chains are really "talkative" and will let you know when they flog out. They give you a lot of warning. I'm only trying to save people from work. :blinks:
Valve stem seals: they're a little rubber seal that prevents oil from flowing down from the top of the head, down your valve stems, and, therefore out your exhaust. To replace them means removal of valve springs, and, to be honest, if you're going that far, you may as well do the whole head. I have a guy in Redcliffe who does all my heads.
Cheers
R