Bit of an FYI, here, the petcock threads on our machines are 1/4" BSPP (British Standard Pipe Parallel) thread. This should not be confused with BSPT (British Standard Pipe Tapered) which is a tapered thread like NPT, though for 1/4", BSPT, like BSPP has 19 threads per inch vs. the much more common NPT's 18 TPI. Confused? Yea, it's silly innit?
BTW, McMaster-carr carries all these taps and dies in stock, though Richard got a great deal on his.