Originally Posted by RFSchaller
I agree with your observation, but look at the application. I wouldn't lose any sleep over more brittle bolts in the application of water pump pressure boundary retention. Landing gear would be a different story.
Rich, put it this way. If the OP uses the specified fasteners he will be 100% safe. If he does not then there could be ramifications that the layman cannot understand. For instance the depth of female thread available may be specifically engineered for an 8.8 bolt and may be insufficient for the higher torque required for a 12.9 bolt. Consequently he may strip the female thread or damage it such that it fails down the track in flight. Neither scenario would good. Alternatively there are other dangers in not torquing a 12.9 fastener to its specified torque.
The fact that the OP is asking this question on this forum suggest to me that he is not an engineer and therefore my recommendation to him is to use the Rotax specified fasteners at the Rotax specified torque.
My other concern would be whether when the OP overtorqued the original bolts he damaged the female threads at that time.