Re: Wheel Spacers...How Thick??
As mentioned, it really depends on the application. A general rule-of-thumb is that you need to have AT LEAST as many threads' worth of engagement as diameter. For example and simplicity's sake, if you had a 1/2" bolt with a 10 thread pitch (10 threads per inch), you would need minimum 5 turns on your lug nuts.
Of course, you will need to do the proper conversion to whatever fractional or metric threads you actually have. Unless they are extra long bolts as on some trucks, most passenger cars don't have that amount of extra threads. I would guess with an 8mm spacer, you would have only 2-4mm thread engagement which is not safe.
Another thing to consider is the scrub radius. If, in the end, you change the center of the tire from the manufacturer's original design, the steering wheel will pull in ruts, bumps, etc., and you may experience excessive wheel bearing wear.