[sterna]

Mecanical parts are the most difficult thing to switch because there are many things defined in relation to one another. For example the american UNC threads have different angles than the metric ones do and therefore it is impossible to replace one with another. For this reason alone it will take very long time to do a full switch from one unit system to another. However, many "metric" units are actually slightly adapted imperial ones. For instance you can get a lot of 25 mm stuff in Europe that is almost equivalent to the 1 inch stuff you get in the US (1 inch = 25.4 mm) and similarly there is a lot of 30 cm and 60 cm stuff in stead of 1 foot and 2 feet stuff.