That's Babbitt bearings--a kind of pot-metal which doesn't hold specs real well...

"a series of alloys used as bearing metal (not just Isaac Babbitt's original formulation). Babbitt metal is characterized by its resistance to galling.

The term "white metal" is also used in reference to bearing metal, but it is ambiguous because of the wide variety of senses that it has been used for over the centuries.

A wide variety of Babbitt alloys exist. Some common compositions are:

    * 90% tin, 10% copper
    * 89% tin, 7% antimony, 4% copper
    * 80% lead, 15% antimony, 5% tin
    * A lead-based based Babbitt (75% lead, 10% tin)
    * A copper-lead based Babbitt (76% copper, 24% lead)
    * A copper-lead-tin based Babbitt composed of (67% copper, 28% tin)

Originally used as a cast in place bulk bearing material, it is now more commonly used as a thin surface layer in a complex, multi metal structure.

Babbitt metal is soft and easily damaged, and seems at first sight an unlikely candidate for a bearing surface, but this appearance is deceptive. The structure of the alloy is made up of small hard crystals dispersed in a metal matrix composite of softer alloy. As the bearing wears the harder crystal is exposed, with the matrix eroding somewhat to provide a path for the lubricant between the high spots that provide the actual bearing surface. Tin gives Babbitt the property of lubrication if run without a lubricant. The friction will cause the tin to melt out to prevent or reduce damage to the shaft.

Combustion motors use the tin based Babbitt. The Tin based Babbitt is able to resist with pounding found in the combustion motor. Lead based Babbitt tends to work harden and create cracks which will lead to the bearing failing.

Lead based Babbitt can be used with constant turning tools such as saws."

(Wikipedia)

A