yes in fxs and fx in uslmra they legal. In fact I run one. They were on simplictys were first who had gx390vs those but most people run horizontals. So you can run either or tho. Compared to my briggs. I make a lot mower power with this clone then that briggs ever did. My buddy run 31ci fully built and I beat him. Only down side about 390 or any clone or Honda is that if something breaks like push rod bents or something simple. You better have spare parts handy with you cause majority of people are racing with brigs.