I do not know the piston clearance. There are guys on here like @dezz and @reogem who can probably tell you. I can tell you what a lot of the guys are going to say though. Get a factory service manual from traidbit.com alot of the members here say it's "worth it's weight in GOLD!" Personally, I don't have one, I should get one, but I am too cheap and stubborn. LOL! They run about $20. I believe they are in PDF format... Not sure. Again others will tell you. You also might be able to find someone willing to send you a copy of theirs (did I mention it's always good to share? Lol!) I personally would love to find someone willing to do that. Hint hint. Good luck with the rebuild!
Its not my quad, I am redoing the motor for a friend so I really dont want to buy a manual just to find the one spec. If someone knows and could tell me that would be great. also I see that I forgot to say its a 450cc.