I had a difficult time replacing a couple newer nipples on my CaseIH tractor. The parts list says 1/8 NPT, but the originals were slightly smaller than standard replacements. The 1/8 NPT nipples I bought could only be turned in 1/4 of the way. Metric were worse with threads too fine. I finally ordered replacements from my dealer using part numbers, and they were no better than the standard nipples I bought. I forced the new ones in using a tight fitting 6 point socket on the nipple, with lube on the threads. After seating them completely, I backed them out and the threads look fine. I'm not sure if the original holes were not tapped completely for the full taper or if I have the wrong sized nipples, but they fit now.