Start with a ½ brass plug clamped into the vice on a drill press. It is important that at no point should you clamp down on the threads. With the plug secured in place, drill through it using a 14.5mm bit. This task can also be done on a lathe if one is available.

       With the plug still in the vice, you can now begin to tap the hole. For a PG 16 adapter you need an m16 x 1.5 tap. To start the tap, apply pressure and turn the tap clockwise until the tap starts to bite. Then back off. Now is the time you should apply your cutting fluid. The actual tapping motion is half a turn clockwise, quarter turn counter-clockwise. Repeat that motion until the entire length of the plug is threaded.

       Once the bolt is completely tapped, there is no need for the square end any more. Turn the bolt onto its side and carefully remove the square head with a hacksaw. Any burs that are left behind should be removed with a file. This is a down and dirty method to make an adapter, but it does work well. Total cost is usually under 2 dollars depending on size, with labor time under 20 minutes. If you do not have a tap and die set, a single tap can be bought for 5-10 dollars.

This site provides an excellent tap drill chart.