Just for some ideas, here's my Savage that linc mentioned:
http://smg.photobucket.com/albums/v132/4x4pede/Savage/
You don't actually have to buy a center spool, you can make your own. I did this when I had my X-Terminator buggy way back when, using a 6mm shaft and a Revo slipper clutch bored out to 6mm, along with a custom spring/tensioner setup:
http://img.photobucket.com/albums/v1...r/IMG_0742.jpg
http://img.photobucket.com/albums/v1...r/IMG_0895.jpg
http://img.photobucket.com/albums/v1...r/IMG_0901.jpg
You can bore out MGT spur gears (46, 49, 52t) to fit perfectly onto the Revo slipper clutch.
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz