I attempted to tap into a pipe plug at the oil filter housing once. I don't believe these are English NPT pipe threads. I believe the TPI is different. If I remember right, this was close to 1/4" NPT which is 18 TPI, but the installed plug was slightly smaller and 19 TPI.
It might be a metric ISO pipe thread.
Here's more info: http://www.engineeringtoolbox.com/thread-standards-d_749.html