If you are worried about ROAR rules, realize there are a few things beside the nuraber that make up the specs. First don't change the sintered rotor that comes with it for experimenting or gain. The standard 12.3mm will be the spec rotor that comes in the motors (unless you go with the sport versions, they come with bonded rotors, the 13mm aftermarket rotor, etc. won't be legal for ROAR rules in anything other than mod), so stick with it. Most clubs haven't totally adopted the ROAR rules yet, so check with where you are racing to be sure since 17.5 is the new stock and 13.5 will be the new 19t. In the past most clubs that allowed brushless (since brushless wasn't legal for anything other than mod for ROAR) used 10.5 for 19t and 13.5 for stock, so make sure you check. Also, timing rings must be fixed and not adjusted (Novak motors have fixed rings), so be careful of that too. If you decide to go with a different brand from Novak, I believe sensorless is still illegal, and a lot of them have adjustable timing; something to consider.