The maintenance manual for the military Chevy trucks reference a roller bearing.
http://chevy.oldcarmanualproject.com/military/1945_46/4x4cmt0203.htmNone of the babbit engines I have come across had the roller bearing. I presume they received bushings after getting new clutches.
Since the pilot bushing is used in a very large number of GM applications over many decades, there are a large amount published and anecdotal references. In general, it appears as though rollers are not recommended for restoration work as they are not as tolerant to lateral misalignment. Old engines that have non-factory/original installed crankshafts, bell housings, clutches, transmissions...have an increased potential for misalignment. The bushing is more tolerant to having a transmission input shaft that is not exactly parallel with the crankshaft more so than the roller bearing.
The National number PB-656-HD is made of bronze and is non-magnetic. National PB-656 (the cheaper $3) version is magnetic. I could not ascertain if either are impregnated with oil but none are packaged in wax paper or plastic wrap and the boxes don't have oil stains so I would conclude that they are not the preferred "oilite" type. This I do not know for fact, but the evidence is pretty strong. Both old style and modern GM (Delco CT1088) pilot bushings are oilite and bronze. Oil is impregnated into the oilite bushings under high vacuum so soaking a dry version in oil may not have the same results.
Metallic bushings are not recommended as they have to potential to fuse to the transmission input shaft and could cause damage. Bronze being a softer metal than the input shaft should not cause harm to the input shaft in the event of a failure.
Non-oilite bushings SHOULD be lubricated with grease upon assembly.
Oilite bushings SHOULD NOT be lubricated with grease as the carrier in the grease can plug the pores in the bushing causing oil starvation. The oil in the pores of the bushing continue to seep out as the bushing warms from friction and should lubricate for the life of the clutch.
For the effort it takes to access this bushing, the choice in materials seems rather evident.
Paul
