Lash Caps

Hey guys, I have a problem with my roller rocker arms on my magnum headed 340. To set the rocker arms correctly, I either have to mill down the pedestals or leathen the valve with lash caps. I've never delt with lash caps before and was wondering how they stay on the valve tip. Are they pressed on? Do I have cut them to fit as I only have .100 between the keeper and the end of the valve. How much clearance from the keeper do I need? Thanks, I know someone here can help me. Bob