I tried to mount 8 cogs Sun Race cassette on the freehub of a trainer which I think is rated as "Shimano" (T2875-01 freehub). The cogs fit ok (with a spacer between the hub and the big cog). However, the lock ring can not be screwed - it is like the freehub has a little smaller internal diameter (maybe 0.1-0.2 mm) or a different pitch. I tried another lock ring, from SRAM 22 (11 speed), but it behaves the same. I try to find a solution and find a 8 speed compatible cassette / lock ring, so I am wondering:

Are the lock rings from SRAM 22 (11 cogs) fully compatible with Shimano 11 cogs cassette? If not, this could explain my issue.
If not, then: are 8 speed Shimano lock rings compatible with 11 Shimano lock rings? If yes, this could solve my issue.

In the meantime, I found information on Garmin / Tacx official website about the freehub supplied:

"Any 8 to 11 speed Shimano or SRAM cassette and standard locking can be used in combination with this cassette body"

This is puzzling, because my lock ring from SRAM 11 speed and the one from Sun Race 8 speed (Shimano compatible, I assume, and similar to SRAM) do not seem to fit.
I didn't force to screw, but now I'm wondering: should I consider that the thread of the freehub is somehow special to require a forced screwing from the beginning?

I agree that some cassette lock rings have a noticeable difference in how tightly they fit within the freehub body threads. I never tracked brands or models but seem to remember that alloy lock rings tended to be a tighter fit on average then the steel more common ones are. BTW I add a drop of oil to these threads before installing the lock ring. Andy

Thanks, that was the case.

In the meantime I had a chat with Garmin support and they finally told me to force screwing and, if the freehub will be damaged, they would replace it for free. So, it took quite a big force to screw; I could never decide on my own to force so hard a new threaded part. I assume it is ok now.

Did you first turn counter clockwise until you felt the threads align and drop in?

Yes, sure. But the thread of the freehub was very tight. I don't know whether it is by design or bad manufacturing. But it works now.

One test will be on the removal of the lock ring. Hopefully no galling or other issues then. Andy

I hope you also greased the threads on both the freehub body and the lock ring. That will make sure things go together as easily as possible and more importantly, will unscrew without getting frozen due to corrosion or galling.

It doesn't take very much damage or deformation to a thread to make a very tight to almost impossible fit. Especially if that slight deformation is on the start of the thread and the other mating thread doesn't get aligned properly.

Also there are all sorts of fit standards for threads of the same pitch and diameter. So some parts with the same threading but different fit standards might be really tight depending on which is inside or outside thread.

It might be a stretch to assume every bicycle part manufacturer uses the same thread fit standard.

Sure, I used plenty of grease. It will stay long time there, since the cassette is on an indoor trainer: no dust, no rain, no other contamination.