This might seem a basic question,

My fuel rail extrusion is 1/2" bore, what pipe thread sise do I need to use and tap the bore to take the feed and relief fittings?

Cheers.

Pete.

[Edited on 22/6/13 by r1_pete]

0.5" is just 0.6mm bit too big to tap for 1/4" BSP parallel ----

If the wall thickness is ok 3/8" NPT might work the tapping dril size is 9/16"