Have found that caps often shows bolt dimensions. This one has 13 so I would take that to mean 13mm thick. I once stacked washers to about 1/2 inch to achieve necessary spacing. And by the way, both my GVR4 and GGSX had this piece from the factory. After I changed an axle and put it back together without it, I ended up breaking a bolt off in there. I borrowed one temporarily from my other car and noticed the spacer there. Then I went searching and found the original in the driveway where I had dropped it while changing the axle. Don't know what forces result in the bolt breaking but they're there.