ORIGINAL: 2slow2matter
ball bearings--support the crankshaft. Most have them at the front and back--this is the best setup. The other alternative is a bushing--basically a polished metal to metal point of contact--much more friction using this setup than with ball bearings--therefore, the ball bearing engines aren't robbed of as much power as the bushing engines are. ---
There is no metal-metal contact with a bushing engine. The crank "floats" on a film of lubricant within the journal (bushing) bearing & has very low friction. The reason BB engines are generally more powerfull is that they invariably have larger & more agressively timed inlet & exhaust ports. Bushing bearings are cheaper to make & are therefore used on low end engines that are primarily employed as "trainer" engines. These can last every bit as long as a BB engine & in some cases are as powerfull as their BB counterparts.