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- Nov 16, 2021
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I noticed that the engine mount bolts were worn on the section that is inside the tube on the engine mount. That got me thinking about the design.
The bolt is a loose fit to that tube. If you torque the AN7 to 5 ft-lb then the tension on the AN7 is approximately 1,050 lbs. With a friction coefficient of 0.2 then it would require a side force of 210 lbs to make the washer slide. Actually a bit higher because of static friction. 210 isn't much but probably sufficient.
However the rubber bushings seem to deform rather soon and that could greatly reduce the AN7 bolt tension making it easy for the washer to slide so that the bolts starts rattling abound in the mount. That would explain the wear I see.
The coefficient of 0.2 is for dry steel against dry steel but this is a fiber washer against painted steel. It could be higher or lower.
Then there's the SB21 1946 service bulletin where engine mount bolts were failing with the A50 was installed.
I'm replacing the bolts, they're dirt cheap but I don't like the idea that they can rattle around like this. The poor rubber composition seems to be a problem too.
Am I off base?
The bolt is a loose fit to that tube. If you torque the AN7 to 5 ft-lb then the tension on the AN7 is approximately 1,050 lbs. With a friction coefficient of 0.2 then it would require a side force of 210 lbs to make the washer slide. Actually a bit higher because of static friction. 210 isn't much but probably sufficient.
However the rubber bushings seem to deform rather soon and that could greatly reduce the AN7 bolt tension making it easy for the washer to slide so that the bolts starts rattling abound in the mount. That would explain the wear I see.
The coefficient of 0.2 is for dry steel against dry steel but this is a fiber washer against painted steel. It could be higher or lower.
Then there's the SB21 1946 service bulletin where engine mount bolts were failing with the A50 was installed.
I'm replacing the bolts, they're dirt cheap but I don't like the idea that they can rattle around like this. The poor rubber composition seems to be a problem too.
Am I off base?
