At Hockey Giant, we get a fair amount of calls from customers who have questions as they're looking for wheels, bearings, axles, and spacers. There seems to be a bit of confusion when it comes to matching up the various parts. We'll attempt to shed some light on the subject by identifying each of the individual parts and the variations that you'll come across as you look at different skates and chassis models.
The core (aka "hub") of the wheel is the center, plastic part into which the bearings are inserted. There are two different core sizes available: 688 micro-bearing core wheels (pictured on left below) and 608 standard bearing wheels (pictured on right below). As you can see, the hole in the middle of each wheel is a different size, therefore they require bearings of different sizes. There are essentialy 4 different types of spacers: Standard 608, Micro 688, Floating Standard 608, and Floating Micro 688. They are labeled in the image below. There are 3 different axles pictured below as well: 6mm Axle and Screw, 8mm Axle and Screw, 8mm Axle Bolt.
The tree diagram below starts with the wheel type (608 Standard or 688 Micro) and then identifies which spacers and bearings will be needed based on your axle type. Most axles, whether 6mm or 8mm, have a female end into which the male screw is inserted, although some axles (usually just 8mm) have threads on the end of the bolt which screw directly into the chassis threads. In any case, the mm size of the axle and the wheel core type determines which type of spacers will be needed in order to properly mount the wheels to your chassis.
Most inline hockey skates are assembled with standard bearing wheels on the chassis, but there are of course some skates that are assembled with micro-bearing wheels. Since micro-bearing core wheels are currently being phased out you might consider making the switch to standard bearing wheels, such as the Labeda Addiction XXX Inline Wheels or Rink Rat Hornet Split Inline Wheels just for the sake of future availability and compatibility.