There isn't a standard 13mm size in afraid, chances are its 1/2 inch, which means it could be bsw, bsw fine, unc, or unf, or mabey npt if its a tapered thread. That probably didn't much really , sorry to complicate things. I have all the charts and a zeus book here at work as I'm an engineer. Can you tell me how mant threads there are per inch?