Originally Posted by SniperDroid
It's either 1/2 or 3/8 NPT. But check with a local dealer, they probably have a hundred of them in a bin somewhere.
X2, but I have never seen them sized at 3/8”.
You need a 1/2” NPT/FIP cap. “FIP” stands for female iron pipe. These threads will be “NPT” which is the American National Pipe Thread standard designation for tapered threads.