depending on what exactly it is, I might work with something like mounting/fastening clip/stud/bolt

Mounting clip sounds good to me. But all the others herem, they got so very used to the term "clamp nipple" that it will be hard for me to change that habit :)

Is is very ambigous to a native speaker`s ears? Or is it just as fine / awkward as the German term is to German ears?

It would definitely cause a couple of chuckles, but if that's what it's called, that's what it's called. I recently had this issue with the term "gaylord" for very large cardboard boxes -- everyone knows that's what they're called, but people still made jokes :o(

However, depending on the shape/material/etc. of these clamp nipples anything from clip, fastening, fitting, bolt would be equally understood.

*F5* Hmm dude is always so fast at answering! I need to stop getting distracted while writing these things :o)

the "other" kind is actually a nipple clamp, not a clamp nipple, but the latter still sounds odd to my ears. And once you put the nipple clamp on, you then have a clamped nipple. :-)

It could sound like instructions...

I'm not that familiar with technical language in either country, but while I've seen the term "Nippel" used often in German for various technical devices, I can't remember ever seeing "nipple" used in English in the same way. (Not that it doesn't exist, just that maybe it isn't used much in paperwork for consumers, for example.)

But "nipple screw", for example, is a very common term for a special kind of screw. That "clamp nipple" is very similar to the nipple screw, only that it is not threaded!