hey stew.. im not an expert in the matter.. but one thing i do know, when you run a one-way.. using your brakes is much more difficult.

the car will spin out if you do not do it right. so most people run a diff so they can use brakes. a spool is the combination of both. you get the same advantages of a one way... but you get to use brakes.

if you need a spool.. check out www.speedtechrc.com. steve has a good one for the 415.

diffs, you can probably pick up all the necessary parts to make one yourself. (see earlier pages for the part numbers) again steve should have the parts (if not, he'll be more than happy to order them for you).

hope that helps!