Have I shared this one before? Sorry if it's a repeat.
The best tap for Silca I've seen is the one Jim Merz made for himself. The reason it's the best is the pilot on the front of it, that fits snug on the inside diameter and forces the tap to go in precisely in alignment.

When he showed it, he said "it's a bit crude...". We should all be so lucky as to have such a "crude" tool!



Pretty sure Jim doesn't offer pump shortening as a free service though!