Thanks. I had a measurement for the top spacers of 1.35" and they're definitely a loose fit so that makes sense. And you're right about the silicone and neoprene. Maybe some of the alternative materials (EPDM, polyurethane, nitrile) would be better suited.The preload mechanism has an od of 1.400" below the rubber o-ring seal. Assuming it is the same part for kashima/black stations, it seems 1.375 would be a slip fit.
At McMaster if you choose raw materials/rubber/rubber/about rubber, there is a chart that suggests silicone would not do well with the fork oil.
No data on neoprene, but it seems like that would get chewed up into particulate by the spring.
Your PTFE guide is making a noticeable difference?