Engineering discussion for the group. Here's the background. For about 9 years I have known about having 1 of the 2 bolts broken on the forward starboard engine mount. These bolts hold a piece of 1/2 inch plate steel which was bent at a 90° angle to the vertical side of the engine. This creates a horizontal shelf. The shelf then rests directly on the vibration dampening mount which is then bolted to the boat. Periodically, I have made attempts to extract the bolt but only a few days ago was I finally successful. The bolt was a M12x1.75. I don't know what hardness the OEM called for. Years ago when I discovered the broken bolt I was quick to replace the remaining good one with one having a hardness of 10.9. For a fresh start, I now intend to replace both of them with new and again a hardness of 10.9. In removing the old bolt, my drill hole was off center and ended up cutting some of the threads. I approximate not more than 1/4 of the thread integrity was compromised.
https://drive.google.com/folderview?id=15GCOH3qKIWXl1lmjsbZqsf-ECGCrl1fN Here comes the question(s). Just for kicks, I decide to attempt to figure out what the sheer strength of the engine mounting bolts are individually. I came up with an answer but would like hear what the group comes up with. I also now have a bunch of other questions tumbling around like: How much weight would it take to break one? How much margin to failure did I have while operating for the last 9 years? What could have possibly caused this bolt to break prior to my ownership? Theories and ponderings are welcome. With 1/4 of the threads degraded on the newly available bolt hole, I plan to only torque the bolt to ~50% of max (90 ft-lbs) which would be ~45 ft-lbs. I'll also make sure that the lock washer is fully collapsed but I'm confident that that will happen well before final torque is reached. In sheer, I believe this bolt is providing as much or more strength simply by being there than anything to do with its torque. All though, I have heard the engineering theory that (when built appropriately) the bolts apply the pressure which creates the friction to prevent the pieces from moving in sheer. Under this argument the 2 pieces of metal don't actually ride on the bolt shank specifically, and the surface area where the 2 pieces touch is of particular importance. Looking at the engine castings, it would appear that maximizing surface area contact was not considered. As such, I can only fall back on my previous thought that the bolt shank is in fact carrying the load in sheer and torque is more or less irrelevant except for the purpose of keeping the bolt tight. I'm also aware of various thread repair options but in the awkward location I am fearful that I would cause more damage than good by attempting any of them. Particularly if the value added by the second bolt truly is its presence not its torque. All thoughts and insights are encouraged. All the best, Josh Muckley S/V Sea Hawk 1989 C&C 37+ Solomons, MD
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