The Alfa wheel bore size is 58.1mm so the central hole in the spacer has to be 59mm.
If your spacers are <5mm thick and destined for the front wheels, then that's all you need to worry about. A regular flat spacer is fine.
If the spacers are thicker than 5mm and/or you want to put them on the rear hub, then they should have a stub welded onto them so they fit snugly into the wheel centre bore... the Alfa stubs aren't big enough to poke through the spacer enough to support the wheel otherwise,
The stub (whether the Alfa one or the spacer one) has to locate tightly onto the wheel centre bore, to support the weight of the car (albeit it also needs the wheel bolts). The wheel bore may be something like 65mm.. and your supplier may be able to weld/machine a stub onto the spacer which is that size.
If the stub is a 5-6mm smaller than the wheel centre bore, then you need a spigot ring. The spigot ring has to be the correct size (inner and outer diameters) to join the two together.
The holes for the wheel bolts are typically 13.5mm since the wheel bolts are 12mm and you don't want to be screwing them into the spacer.. just pass straight through.. and the PCD is 5 x 98mm (that is, each hole centre is on a circle 98mm in diameter... and you have 5 of them on the GTV).
Ralf S.
