Spun rod barron

You can do it from the lower end of the engine-think cleaniness as you do it-

Usually not. If the bearing is simply worn excessively and knocking it is possible to replace crankshaft and connecting rod bearings without removing the engine, but most of the time the "journal" the bearing goes around is chewed up too. That will destroy a new bearing within the first few revolutions of the engine, in less than five seconds. The proper repair requires replacement of the crankshaft or it must be ground down to a standard undersize of.001", .002", .010", .020", or.030". Special equipment is used to do that at engine machine shops and the work is very precise. To remove the crankshaft for that service requires removal of either the engine or the transmission. On some engines, older Chryslers for example, it was a little more work but the crankshaft could be removed without removing the engine or transmission, but GM engines and most newer engines have a one-piece rear main seal that can't be removed without separating the engine from the transmission.