"Right now this screw is worth exactly the selling price of the whole motorcycle ..."
This is not marginal utility as somehow the value of this screw is equal to the entire amount of the motorcycle. There is no marginal utility to be had because decreasing it or increasing it has no effect. This would not be a continuous function. You could talk about the marginal utility of one having a motorcycle or no motorcycle. It's not related to the increase in screws.
Additionally, to say that expertise in a subject is not needed is perplexing to me. So much so I literally can't begin to understand how you'd surmise such a claim honestly.
How much economics did you take? I ask because it sounds like your understanding is that from a class or two, and not the bulk of your education. I think if it was we would not be having this discussion or you'd somehow prove it instead of reiterating the same points.
I don't see it as productive to respond on the internet to the posturing and credentialing about who has seen more utility functions or taken more econ classes, as you wrote above.
I can assure you that Robert Pirsig garners utility at different costs and quantities in the scenario he described. That is all that a utility function is: a function of utility at varying quantities. This is an elementary topic and there's nothing more to be argued here. I'm sorry that explaining it again rubbed you the wrong way and encouraged you to write a condescending response towards me.