SetFree93 - 11/21/2024 12:26

Here is my working theory:

I suspect that the factory precisely machines the cast iron bearing bores before they encase them in vulcanized rubber. The original bearing bores are perfectly round.

The vulcanizing process occurs at around 284-356 degrees F. Vulcanized rubber grows and shrinks around 15X as much as cast iron, (for engineering nerds, this is a function of the coefficients of linear thermal expansion). This means that, as the hot, rubber-encased bearing housing cools, the rubber seriously squeezes the cast iron. You want that to happen to get powerful bond between the rubber and the cast iron, but all those squeezing forces distort the cast iron.