'81 KZ1000J inner damper fork tube tool

I just rebuilt some GPZ1100 forks for my ELR clone and I got one bottom bolt on the fork leg that is spinning a bit and I would like to make a tool for the allen wrench slot in the damper tube to hold it in place while I torque the bolt. What size allen wrench will fit in the damper tube?. Looking down the fork I can't tell if it is 6mm or 8mm or??. Thanks in advance for your help.