Dunno. Not looking for a fight over this, just want to help the guy with my own modest, limited experience, which includes frying one Rossi 81 and one Rossi 65 small heads in my first Byron F16 before I got the right engines.
I guess OS and all the other engine manufacturers felt it was important enough to make two head castings and two versions for all their ducted fan engines, and while YOU may have had luck running Byron fans without the larger head, I think OS, Byron, and the majority of Byron pilots would just say to get the larger head.
Anyway...it's not something worth fighting over, is it? You can have your opinion, too.
Good luck, viper. What is the VX going into?