I had to acquire a 3/8 BSP tap to tidy up some of the threads that are cut into the crankcase. These are the water drain tap hole, both oil sump drain plug holes and the mounting hole for the post that supports the governor mechanism.
With years of paint, rust and grease in these holes, not to mention what I had added (with paint etc), they needed cleaning out. Unfortunately I didn’t possess a suitable holder for the tap, but as you can see from the photos, a pair of molegrips were an equivalent substitute. All these threads were cleared out, ready for their intended component, which will hopefully be a snug fit!