I recently read an article about Polybush, the firm that makes polyurethane suspension bushes. In it they explain that the up/down movement comes from the flexible material in the bush, not from the mounting bolt and/or sleeve. I know this should be obvious but some recent postings have suggested that there is some confusion about this.
This is why the suspension is supposed to be at normal ride height before the bolts are fully tightened.
Incidentally most of their bushes are cast rather than machined, although they do supply material for machining if required. They use polyurethane because it can be stretched 500% without tearing. Could also explain why they are so expensive
John