Hello Mike,
In order to analyse the load bearing capacity of the preloaded bolt, I would model it in much simpler way, reducing my computational effort and also avoiding any potential unwanted behaviours like you're experiencing. You can get rid of those threads, and include a simple cylindrical geometry. You can model the bolt as a line body using beam connections. And apply preload using in-built bolt pretension tool avaiable in mechanical. Have a look at this Ansys Innovation Course, which guides you to model your case more accurately, yet using less computational effort.
Modeling the Bolt and Preload | Ansys Innovation Courses
Connecting Bolts with the Rest of an Assembly | Ansys Courses
Regards,
Nanda.
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