Here's my list of specs on aftermarket sway bars.
One very important note, the EVO rear bar's different shape is also to allow clerance for running true dual exhaust setups.


Stock oem sway bar diameter for the Z, G35 coupe and the sedan with sport suspension is 33mm front 20mm rear. I have every reason to believe that they are all in fact the same part.

Nismo:
front- *36mm
rear- *22mm

*However this same person got different measurement's than I got when I used a dial caliper on the stock bars. So I have to question the accuracy of the above measurements. But their all I could find. No offence to the man that posted his measurements.

Cusco sway bar
35mm front
22mm rear
Front sway bar has two settings of 20% and 37% stiffer then stock Rear sway bar is 73% stiffer then stock.
$450 shipped

Stillen 5-way adjustable bars
35mm front 22mm rear (converting from 1 3/8" & 7/8")
Adjustment range 7% to 70% (unknown if both front and rear bars have the same full 7% to 70% range)
$479

Mines Sway bars
Front 20% stiffer rear 30% stiffer
$699.00(OUCH!)


Eibach specs from Eibach.com 34mm front 22mm rear
ETA 09/01/03 $264 from www.tirerack.com

350EVO Sway bars 3-way adjustable
32mm front and 23mm rear.
$495
Front bar
middle adjustment 11% stiffer
stiff adjustment 33% stiffer

Rear bar
middle adjustment 25% stiffer
stiff adjustment 53% stiffer

If I missed any known sway bars please post a url link so I can research it.