ANd be careful with that...1/8" NPT, contrary to common sense, is NOT 1/8" in diameter. Make damn good and sure you are purchasing 1/8 NPT (National(?) Pipe Thread). You also do not want the British pipe thread size.
You will prolly have 2 or 3 petcock styles to choose from...some are radiator-style petcocks, others are intended for air bleeding (but work fine).