How do I change a cam belt on a cavalier 20i 16v?

best thing to do is buy a camshaft pulley locking tool, I got a draper one for under £5, specially made for this engine, it slots between the pulleys when the timing marks are lined up and keeps them in place (without it, the valve springs turn the camshafts and make life very difficult).
changing the belt is fairly simple but you need to remove at least one engine mounting. Getting the timeing wrong causes serious damage to valves and possibly camshafts which I've seen shattered in this event.
Another thing well worth buying is a haynes (or similar) manual which will show you all the timeing marks and "how to" step by step, and the tightening sequence for the head.
It is also important to check the tensioners and water pump for any noise when spun by hand or play (and stiffness), and leaks from the water pump. Ideally these should all be changed anyway but the water pump should be fine for up to 150, 000, I would advise you to change the tensioner and idler pulleys though as they're not expensive and are the most common cause for a broken belt.

Good luck

Richard weaver
December 2004