This going to sound really stupid but I bought my son a Nosram Storm 6.5 brushless and at the same time I bought myself the LRP X11 6.5 as my LHS didn't have two of the Nosram's.
No problem I thought, same motor different wrapper, until I came to use it and found the motor shaft on the LRP is shorter than on the Nosram motor. I had to buy the long boss pinions to get a good mesh when using it in my Predator, although I've now moved to a Cyclone D4 and don't have the same problem with that.