Heres a pic of my 147 turbo with 12mm spacers on the rear to fill out the rear arches.The wheels are 8.5 x 18 inch alloys and the wheel sits 12-15mm out past the arch itself,you can see this in the pic.The rear arches are also rolled and flared out too to accomodate the wide wheels and spacers.Mostly for a nice show look.
I only use spacers for shows and not road use and I use 60mm long wobbly bolts to fit the wheels through the spacers to the hub itself.The spacer is specificly made for the 147 hub and wheel pattern and bolts onto the hub with an 8mm counter sunk bolt to keep it in position,then the wheel sits onto the spacer and is bolted to the hub itslef with the longer wheel bolts.
Roswell,-wheel spacers are perfectly safe to use as long as they are securely fitted and fit onto the hub of the car and as long as there is enough of the bolt thread screwed into either the spacer itself or through the spacer and into the hub to secure the wheel.
Some race/trackday cars use spacers to widen the track of the car and give the car better handling and feel to it.
The thickness is the last thing I need mate the overall dimension - between the holes, center hole diameter and the diameter of the whole spacer....I want to make them myself if this helps with what I need