It could be a fine metric thread M1.6x0.2 or M1.8x0.2. I don't think M1.7 is standard.
The pitch is close to 0.25 - it is easy to measure on the picture. 0.2 would be too fine. The diameter is around 1.65-ish
It looks to me to be a fine thread for very small precision movement inside a camera. It could well be a special thread for this type of task common in the camera industry.
I think that's what it is. If many of these motors come from Cannon, they're most likely used in cameras. I guess chances of finding a nut (let alone tap) are slim. It would be nice to figure out the name for this thread, so I could search for nuts or taps.
As they come with a nylon nut already on the thread it would seem sensible to just use this. Why worry about replacing them unless you have to?
This nylon nut has a weird shape and I don't think I will be able to use it.
Some of these are sold without nuts and hundreds of people buy them (according to eBay anyway). Apparently, these people are able to obtain nuts somewhere or cut the thread by themselves. I thought this was a well-known thing among robot makers.