got it tight with a ratchet then used a torque wrench which had been calibrated recently so it was correct, the wrench spring hadn't started to move towards clicking when it snapped.
When it broke and when I looked at the metal it was a ver porous casting
Replaced it with a NOS U Bolt and that was done up in the same way, no snapping and I re used all but one of the original bolts doing them up to the same torque value.....guess what? None of them snapped either