Make absolutely sure it is the screw head, and not the tool that is stripped. Quite often, on the smaller screws used on our engines, there is very little room for a sloppy or worn tool. I will usually try to grind the tip of the allen wrench off about 1/16 - 1/8" to get a good clean tip. Don't try to use a "ball end" driver. You can also sometime use a slightly larger wrench, maybe if it's metric, use the next larger SAE size, or vice versa, that you can force into the socket.