flywheel torque to yield bolts

[inaminit]

In another thread on this forum is a discussion about flywheel bolts and the correct way to install them. Some of the posts made me stop and think a bit about why such bolts are used in this particular application. The challenges to the bolts are shear failure from excess torque, loosening from vibration or from the rotating parts being out of plane with one another. So what is the advantage of these particular bolts? Thanks in advance for your answer.

[M Baldwin]

I found myself asking this same question while doing the flywheel on my 986 recently. I did a little reading and found that the idea is to get greater clamping force from a smaller fastener. If my understanding is correct, tightening to "torque" puts the fastener into the elastic portion of the strain curve. Tightening to "yield" applies more clamping force, but we are now into the plastic zone of the yield curve.

In the elastic zone, the fastener will return to it's original shape. Once you cross the line into the plastic zone, the fastener is permanently deformed, and cannot be reused. The key, of course, is not to go so far that we hit the failure point.

[JFP in PA]

Newer engines require high clamping forces which cannot be achieved with the smaller diameter bolts normally found in these engines. Torque to yield fasteners provide more consistent clamping loads across the entire mating surface when using smaller bolts.