I make sure when I disassemble a cap, the top end with the clip comes off first, using the largest punch that EASILY fits through the threads (it should wiggle a bit...). Then I use the largest punch that will fit down the tube, This one should be as snug as possible but still move freely, you don't want to distort the tube, but you want to be sure you are pushing the threads out evenly and with as much of the force on the leading edge of the plastic insert, NOT just the threads. I also use a hammer on the punch and only hold the cap with my hand. It often takes many hits with the hammer to fully remove it, but by holding the blank in my hand it allows it to slip a bit so the insert doesn't get too much force exerted on it all at once. I've never had the insert damaged this way...
