Originally Posted by engineczar
To go along with what BBC has said when your doing a cylinder make sure the piston on the cylinder you're doing is at top dead center, that way if something happens the valve will only drop until it hits the piston and you won't lose it down the cylinder.
The compressed air will drive the piston down to bottom dead center unless you have it in gear (standard trans). If it's an automatic there is no way to keep the piston at TDC short of some mechanical means. The piston not being at TDC has never been a problem for me as many as I have changed over the years. Also give each valve retainer a smack with a hammer before you start, it will make the retainers easier to get out.