Here is an 8 piece kit to convert the normal after market shift knob from 16mm to U.S. common thread sizes. We made our inserts from brass to eliminate seizure, which is all too common with aluminum. These are now slotted on one end (not shown), for usage of a screwdriver to remove adaptor from knob - a very desirable feature.
The included shifter threads are:
3/8"X16, 3/8"X24, 5/16"X18, and 1/2"X20. Included are jam nuts and instructions.