I just a bought a grade 12.9 (I think standard was 8.8 or 10.9) unbrako cap screw from blackwoods it hasnt broke yet I have had it in nearly 6 months now. Im pretty sure it is an M16 x 120mm. I have pics from when I did mine. Although mine has held up, I dont know if a higher tensile bolt is actually the answer, a higher tensile bit is actually more brittle which is more prone to shearing as my original did. By the way I only paid $4 for the bolt as it is only a bolt, dont get ripped off by someone telling you its specially designed for an xr6 turbo diff.