I have to replace an M14 class 10.9 (SAE grade 8, AIUI). I can get the bolt locally. But, at the same store, or anywhere ex online, the highest grade nut available is class 8.8 (SAE grade 5).
So, my question really is, can I use the class 8.8 in this situation. But, more generally, is it the case that the bolt needs to be stronger than the nut - maybe it's under more stress because it's longer?