Hard to say which it is, but there are 3 sizes all about the same physical size, 1/8" NPT (0.407" OD, 27 TPI), 1/8" BSP (0.407" OD, 28 TPI) and M10x1.0 (0.394" OD, 25.4 TPI) <- this is what the brake line fittings use.

I have made use of a greased 1/8" NPT pipe thread tap to modify threaded holes, like for my diff. breather fittings. They are not a high pressure fitting and with a bit of Teflon tape, they seem to seal up just fine.

r.c.brown@ieee.org
TruckEditor@tlca.org
Project: 4Crawler
4Crawler OffRoad - 4x4Wire discount code