Had a look on google, Maryland Metrics Thread Data Charts: Extended thread size range (ISO)
20 mm 2.5, 2, 1.5, 1, 0.75, 0.5
Ordenary range:
20mm 2, 1.5, 1
Seems like plenty of choice, didn't realise there was so much, the extended range chart goes from 0.25 mm, to 600 mm. Ian S C