Thanks Desertfox;
First, let me clarify, we are working on a torsion bar spring, not a coil of any sort.
second, I understand;
T/J= GQ/L re-arrange to find J
TL/GQ = J (but I disagree with your result 6.51x 10^-5)
100x18/(12x10^6x22 x2x3.142/360)=6.51x10^-5 inch^4
shouldn't it be>>...
100x18/(12,000,000x22x2x3.142/360)= 3.91x10^-4
therefore TR/J = t now using t =76000
100xR/ 3.91x10^-4 = 76000
now transpose to find R
R = 76000 x 3.91x10^-4 /100 = .296896 inches
so the diameter would be 2 x .2969 = .5938 inches.