The size of the swingarm spool you will need will depend on your particular bike.

you may want to check your bike and make sure you have the (6mm, 8mm or 10mm)  threaded portion on your swing arm to mount them.

Typically:

Yamaha use a 6mm Swingarm Spool
Suzuki use a 8mm Swingarm Spool
Honda use a 8mm Swingarm Spool
Kawasaki use a 10mm Swingarm Spool