Regarding the pressurization question raised by Earle, the circumferential groove and partial parting line relief on the inserts shown here is substantially different than the stock, unpressurized, rod bearings.
The stock rods had X grooves across the rod and cap, with a V groove across the entire rod at the parting line relief and continuing across the thrust face. These carry oil across the entire bearing. There is also a "faired out" scoop at the oil inlet dipper hole that directs oil to the center of the bearing rather than around the entire bearing which the single circumferential groove does. The stock grooves will spread the oil across the entire bearing and allows seepage across the thrust face.
As a general proposition, the circumferential groove is for use in pressurized systems because unpressurized oil will only seep about 1/2 inch from the groove, about one inch total, less than the 1 5/8 inch width of the A rod journal. This may help explain the uneven wear and ring that Eystein experienced. The parting line relief in the inserts shown here may alleviate that problem, and perhaps Eystein could tell us if his inserts had that parting line relief. If not, it is likely that there was inadequate lubrication on the sides of the journal removed from the circumferential groove.
Another question I have about the single circumferential groove is that the pressure generated when the dipper hits the oil in the tray (about 20-30 lbs at speed) is immediately lost by escape through the holes in the rod big end that spurts oil upward into the cylinder area, and is asserted by Murray Fahnestock to collect additional oil from the mist on the upward stroke. Fahnestock was close to Ford, and his thoughts may reflect the thinking of the Ford engineers.
Nor would I use a MOdel A rod without a side thrust flange, with all due respect to Mike K. Removal of that flange will add about .100 to the rod end play, rather than the .008-.012 called for in the specs. With .040-.050 end play on the pin, that rod will migrate back and forth across that distance during operation. I doubt that modern rods have that kind of end play, with or without an insert flange.
I personally do not use circumferential grooves on my unpressurized system. It would seem to me that the manufacturers could make inserts, rods and mains, that mimic the Ford grooves which have an 80 year history. Babbitt failure is much more likely the result of improper babbitt installation than inadequate lubrication if the stock design is followed. (Ask Herm his opinion about babbitt installation!) Unfortunately, most babbitt bearings I have seen do not use the Ford groove design. One of the arguments in favor of inserts is that the quality of material is fairly uniform and good, while the quality of babbitt material and installation is uneven at best.
I look forward to seeing the results of Dave in Mn's tests and appreciate his willingness to be a "guinea pig." The Model A is simple and durable enough that about anything will (and has) work for awhile. But as noted, I would feel much more comfortable using unpressurized inserts if they had the Ford groove design. My opinion notwithstanding, this is a good discussion of an important issue that is short on facts and long on opinion. Inserts are probably the wave of the future because quality babbitt installation is getting harder to find and the guys that do it are usually some long distance away. If that is the case, I would like to see the insert manufacturers get the best design. I personally do not think that a modern groove system premised on pressure oil is applicable to an unpressurized system, but I am more than willing to see what the results prove. Unfortunately, it is going to take some time to get reliable results.
I have tried to upload some pics but that has not worked. Will try again later, or maybe find a 10 year old to show me how to do it. Or perhaps someone with more computer knowledge than I can help us out here. My article in the January 2010 Restorer has pics and more detail.
