@ Asker 20:36 Apr 20, 2018
Your own suggestion, I'm afraid, takes a too simplistic view, as if a 1:1 word-for-word correspondence were always possible, as well as overlooking the very common word order reversal between EN and FR.
You cannot simply take 'sealing' in isolation, and imagine it is some kind of physical object: 'a sealing' — the word in EN is uncountable, and refers to the action, result, or purpose of making something water (etc.) tight.
Here, you have to keep 'sealing olive' together as a whole — it's 'an olive whose purpose is to achieve sealing'; at best, it might be 'olive d'étanchéité', though as we now know, there are other options; basically, as this is pretty much the only normal function of an 'olive', it is largely unnecessary (indeed, even in EN) to specify 'sealing'. | 
peer agreement (net): +4