It's good to know that the M12V series has the same size as the 1617.
The ebay listings for the M12 collets have photos that says they are 20mm.
The Amazon listing for Big Horn collets that are supposed to be compatible with the Bosch 1617 (and other models) says it's M19x1.
For what it's worth I downloaded photos of Bosch collets and measured in Inkscape. One photo, from a repair site, had a nice top view with a 1" calibration grid. That gave me 19mm for the inner thread diameter, which corresponds to 20mm outer thread diameter, or M20. But unfortunately it was a low resolution picture so I am not confident I was measuring the right parts of it.
I also got a high resolution photo of the collet from Bosch's website. Unfortunately it was tilted, so I had to use trigonometry to estimate the tilt (24 degrees) and use a 1mm thread pitch to estimate the scale. This gave me 18.3mm inner diameter, which presumably corresponds to 19mm outer thread diameter, but there was no doubt a lot of error due to the use of thread pitch for calibration (hard to measure and need to correct for photo tilt) and perspective which I did not take into account.
Last edited by arpruss; 05-02-2019 at 10:35 AM.