[Dave in MN]

Synchro909,
I have used AER bearing shells for the past 12 years in over 180 engines. Since I started using them I can only recall 4 engines that had the experience of the bearing material breaking loose from the steel shell like you describe. It was determined in all cases, the problem was too much advance while running under a high load or speed. One gent had his timing north of 40 degrees advance, the others were well over 30 degrees with higher than standard compression heads. In three of the cases, the repair was to polish the mains, replace the inserts and set the timing correctly. The fourth crank needed to be turned to the next size smaller.

In all cases, the engine also had insert bearings in the rods. The rod bearings showed excessive pressure marking on the inserts so they were also replaced as a precaution. The engine with the damaged crankshaft showed little bits of bearing material deposited on the rod bearings. Apparently, the oil carried some of the pulverized center main bearing material into the rods.

During this same period of 12 years while taking engines in for rebuilding, I have seen well over 75 percent of the engines with Babbitt bearings broken up and delaminating. That’s not to say that there is a problem with Babbitt bearings though as it would be normal the engines brought to my shop have bearing problems. The high percentage of failure exhibiting bearing fracture is typical of Babbitt because the clearances increase to the point where the oil film cannot support the loads and the crankshaft journal comes into contact with the bearing material causing it to fracture.

I specialize in installing insert bearings in engines. I do not pour Babbitt bearings. If someone wants Babbitt bearings, I refer them to another shop. If there was a shop pouring Babbitt correctly, close to me, I would be using Babbitt if the customer desired. I think either Babbitt or insert bearings will work well in our Model A or B engines if installed and fitted correctly. The debate continues as to which material is more suitable, I don't see the end to the debate anytime soon. I think of greater importance than the choice of Babbitt or insert is the precision of the installation. For those readers considering Babbitt or insert bearings: Choose a shop that is the best at what they use and one that stands behind their work. Keep in mind, oil clearance is critical for either type of bearing. Too much clearance and the oil film will be lost at high loads. Too little and the bearings run hot and are in danger of seizing if inserts.

Synchro, leading up to this failure, you know there was a “cross-fire” problem between cylinders 3 & 4 because you heard the pinging. Now that you are familiar with the cross-fire problem, you would be very quick to recognize it should it happen again. You understand the importance of proper timing and know how to set it. Lastly, you know to retard your timing slightly when pulling the camper up a long hill. I believe your bearing failure occurred during the cross-fire event. I also believe you would have done damage to any bearing material during this event.

You have gotten almost 40k trouble free miles on a set of insert bearings and would likely still be running if it were not for the cross-fire event. If the engine were mine, I would polish the journals, replace the inserts, make sure you have about .002” oil clearance at the rear main and run it. With the additional bearing width at the rear main, it can handle and actually needs the additional clearance. The pressure oil you feed to the center main is a very good solution to keep this often abused bearing well oiled. Keep your timing in check and you will very likely greatly exceed your previous 40k. (I have 98k on my Phaeton insert bearings with pressure oil to the center main and filtered oil.)
Sorry for the length of the post...it is long but hopefully helpful!
Good luck with your project and Good Day!