What is the difference?
 

Between a 1515 Neu and a 1521? Thanks

length. check out castles website.

the longer they are the more "powerful" they are, and are suitable for heavier loads (depending on the kv of course). eg: a 1506 is fine for 1/10, 1509 is for light 1/8 buggies and hugely overpowered 1/10 things, 1512 is for 1/8 buggies, 1515 is usually used in overpowerd buggies or monster trucks/truggies etc, and 1521 is just complete overkill for most things.
hope this helps :)

So a 1521 could be used in a Revo. Would fit?

no the 1521 is to long, the biggest motor that will fit on a revo is a 1515, i have a 1515 2.5d in mine and it is masive over kill

I have a 1.5 in mine and it is overkill as well on 4s. I had a buddy of mine asking about the 1521 and I was not familiar with it. Thanks for the info.

he must be crazy!

A 1521 would be killer in an LST on 6s with a MMM... Or the big XL savage.