What is a wheels centre bore
The centre bore of a alloy wheel is the hole at the back of the alloy wheel that fits over your hub.
Most modern car wheels are now fitted with what are called hub-centric wheels. This basically means that the hub protrudes out so that the alloy wheel can fit tightly over the hub. This method allows the centre bore to become load bearing, this result in far less stress on your bolts or studs.
An ever increasing amount of alloy wheels have a centre bore larger than what is needed so that they can fit a larger range of cars. Spigot rings are then used to fit inside the centre bore of the alloy wheel to make the correct centre bore for your car.
Some cars have lug-centric wheels which means that your bolts or studs have a lot more stress being put thro them as they are fully load bearing.