Hey Kevin- Babbitt is made from lead and tin and maybe a little copper I think. It comes in solids "bricks" or ingots and must be heated up to be molten enough to pour. It can then be poured into somewhat rough forms placed around the rod or main bearing journal areas. After it cools, it looks similar to lead. It is soft enough to scratch easily. It can then be machined to size and shape. Rod clearances I would guess to be in the .0015-.002 range. Shims were typically added on the parting lines of the rod and cap or block and cap surfaces, so that after the babbitt wore in some and got a little loose, it could be easily adjusted to be tighter again. Most of the wear occured up and down and not side to side on the bearing surface. Modern insert bearings are a copper shell surfaced with a thin coat of babbitt type material. They are easier to make, easier to change and seem to last fairly long. Poured babbitt makes a good bearing surface, but takes someone experienced in it to pour and machine and adjust. I think this added up to extra expense and labor for the car companies and they replaced babbitt bearings with snap-in style bearing inserts as soon as they could. Babbitt is still used in some large industrial bearing applications.
