M25 standard outer diameter should be 25mm, but actually it is generally smaller (so as to facilitate assembly), about 24.8mm.
"Tooth gauge measurement 16", is it wrong? Is it 1.6mm? If it is 1.6 mm, it should be a measurement error, and the standard is 1.5mm, because the standard pitch is not1.6mm..
There are 16 teeth per inch, so the pitch is 25.4/ 16= 1.6mm, but it is not in the pitch standard of 1.6mm. It may be 1.5mm or1.