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Traditional rear hubs came with a standardized set of threads to which a standard freewheel/sprocket cluster could be screwed on.
This allowed any brand of freewheel to be mounted on any brand of hub.
If you wore out your sprockets, or wanted different gear ratios, you could unscrew the cluster and install a new one.
Beginning around 1980, the Shimano "Freehub" largely replaced the conventional threaded rear hub.
5- and 6-speeds used 3.65 mm spacers, 7-speed generally 3.15 mm, 8-speed 3.0 mm.Sprockets in many cassettes are held together by three small bolts or rivets for ease of installation.These bolts or rivets are by no means necessary, they just make it easier to keep the sprockets and spacers in the correct order and position when they are removed from the ratchet body. Some of the high-end cassettes use a "spider", an intermediate metal casting, to hold 2 or more of the largest sprockets.There is a special splined tool that fits the notched hole in the lockring.
Some lockring tools have a long handle, others, like the one in the photo below, have a hexagonal fitting like a nut, which can either be turned with a large wrench or clamped in a vise.
Removing a freewheel is a chore, because pedaling tightens it onto the hub threads.