CF bolts which material

Hi got our X1E last week and printing CF PLA and PET CF , really impressed with the finish
I need to print a lot of 1/2” bolts , custom design with a thin head, and Allen key fixture , printed at 100%
I’ve printed 6 prototypes and the thread works great, when i torque it a bit the head snaps off
The hex was 6mm and 3mm deep , bolt head was 3mm thick
I’ve redesigned it to 4mm hex , 3mm deep , and the head to 5mm
Printing tomorrow
Any thoughts?
Would CF nylon be better?
Thanks Dean