try being here then, you buy some 15x21 metric fittings, unbox them and 9/10ths of the time cast into the side is 1/2", same for 3/4 etc.
Only you cant get 10 or 15mm pipe, only 12,14 and 16. So the 1/2" fittings mate to flanges which fit onto either 14 or 16 mm pipe...
The joys of metric
