Emery paper and a drill can be an easy way to reduce a thick nipple. Put tape over the threads of the nipple and then put that in a drill. Hold the emery paper around the thick part of the nipple and SLOWLY run the drill. Check the nipple from time to time to see when enough has been taken off.

Also if the caps are not fitting good, try some Remington #11 caps. Or all that might be necessary is a file and take down the very top of the nipple. Sometimes they flare a little with age.