My guess is that the original 8.8 bolt will be a mil spec product with cad plating. Cad plating is very lubricious and has a lower coefficient of friction compared to unplated steel. 12.9 fasteners are normally black steel due to the quality control problems associated with plating such a high strength alloy. Of course using a black bolt has it own shortcomings in terms of corrosion. But all other things being equal, the black 12.9 will have to be torqued to a higher level than the cad plated 8.8 to achieve the same clamping force. Now one could go to Milspec-HDBK-60 to get a detailed formula for calculating the torque based on various friction co-efficients. But I don't think the OP is going to do that.
So under the circumstances I still think that the conservative approach for the OP to take would be to obtain the engine manufacturer's specified bolts with the same plating and use the engine manufacturer's specified torque.
You’re only as good as your last landing
Last edited by Captain Avgas : 11-17-2015 at 07:55 PM.