When I installed my new engine (replacing the XP-400) I used all metal lock nuts. When I originally installed the XP-400 the geometry of the case where the nuts go made it next to impossible to torque the nuts AND get the cotter pins in and bent over because one had to insert the cotter pin then rotate the bolt 180 degrees to be able to bend the cotter pin over. Inevitably the bolt would turn but the nut wouldn’t and the cotter pin would get jammed against the nut. When I removed the XP-400 it was a real bear to get the cotter pins out. That’s when I decided to go with all metal lock nuts/undrilled bolts when I mounted the new engine which was built using my XP-400 case/sump and all new everything else.
This isn't something unique to your XP-400. It has always been challenging on any 4 cyl dynafocal Lycoming style engine.
Design changes on RV kits aren't only made for things that service history shows to be necessary (usually issued with an SB or notification). Far more often they are made as improvements to a kit in areas that a lot of builders are challenged with.
That is the reason in this case as well.
Changing to self locking nuts brings the RV-14 kit in line with what many certificated aircraft have been doing for w while now, rather than just following the path of tradition.
When a change
is made as just a general kit improvement, it will not usually be issued under a Notification or SB because it is not considered to be for a potential safety of flight issue, or performance improvement of the part(s). It is likely that it just makes things easier or it is a substitution for a part that has become obsolete or what we consider to be excessively over priced.
So the next question likely to be asked is why hasn't the hardware call out been changed for all the other models?
The answer is that it would steal far more resources than anyone outside of Van's can imagine, to make changes to all of the different packing lists, and plans pages for all of the different models.
In this instance, an improved version of the FWF kit was being introduced so many packing lists and plans pages were already being revised. That makes it a very simple process in this case.