Dave, shouldn’t you include the ’54 in with those full pressure engines rather than ’55 and up? May be I just miss understood what you were saying?
Most manuals or papers will describe main bearing fit as “selective fit” which means that shimming may be needed. The ideal starting point is a perfectly line bored block and a perfectly straight crankshaft. This is an excellent paper from Clevite to establish a starting point.
http://www.mahleclevite.com/techbulletins/CL77-1-205R.pdf But nothing is perfect so that’s where the selective fit comes into play. A couple of tenths here and a couple of tenths there and before you know it the crank is binding up so you add a shim till it turns free.
Panic, yes the question often comes up about shimming which results in an oval bearing bore rather than a perfectly round bearing bore. You will notice that main bearings will show most of the wear on the bottom (cap) then the top bearing shells because that is what takes most of the load. The rod bearings will show more wear in the top shell then the bottom (cap). Because the majority of the load is applied at these points their clearance is most critical. These areas need the most support from the lubrication to avoid metal-to-metal contact. Since the loading is less on the side or at the parting line you can live with less clearance therefore less thickness in the oil film.
Denny Graham
Sandwich, IL
