i fitted a 21 crank in my 18 as i got it new from fire foxracing . they were supposed to sell me a new crank for my 18, but due to a mix up it was sold to some one else. to remedy this( as i had paid) they had a new 21 crank which they sent me . this obviously didn't fit so i sent it back and they machined 2mm off it to make it fit!
runs fine and had no probs with it at all!
maybe as the 21 crank is now lighter than standard it spins up faster.