I know that Rossi makes a .21 that will produce 6.86 Lbs on the RK-720 fan, but will not fit the RK-20, if you have the RK-20 you can get the mvvs .26, the K&B .21 or the OS .25 VF/F, it depends on which fan you have, i have a OS.25 VF/DF I'll be willing to sell for 90.00+shipping, will fit both fans