Just finished rebuilding the engine with the Zenith Stomberg head converted to Weber inlet manifold.
The engineering shop forgot to drill and tap the hole for the brass tube servo vacuum take-off.
The thread is 1/8" NPT (National Pipe Taper). My local tool shop had the tap, I took the alternator pulley wheel off, stuffed kitchen roll tissue in the inlet manifold, made sure the inlet valve was shut, drilled an 8.5mm hole, and the tap went straight in with a bit of WD40.
The brass vacuum take off union fitted perfectly. I thought that was mission impossible. Result.