Main Bearing Issue

Ford 841 172 Gas Engine. Approx. 3 yrs. since I rebuilt engine and performance has been flawless. The last couple of uses I have seen my oil pressure fluctuate from 40 PSI to nothing. I had to get out a round bale and my pressure went out and I started getting a knocking. I dropped the pan, oil pump, ect. and started inspecting my bearings. All rods and mains were plasticgaged and checked out fine. All bearing were soaked in oil when removed and in good shape. Since they are out I will be replacing anyway. My question is what causes the damage to the outsides of center main bearing? Could it be that the crank is banging back and forth? There is a little daylight between the edge of crank journal sides and outside face of main bearing. Could this be the knocking sound I here? If so is there a cure? Right now my crank is in engine and I would prefer not to remove.
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May not be related but I saw this once before on a ford pickup the company I worked for had. After about 1 or 2 hundred thousand miles it started knocking. It was the torque converter bolts hitting the block. Seems that all those years of the automatic transmission fluid pressure pushing the torque converter forward wore down the thrust surface of the main bearing in the crank. New set of bearings and it was good to go for the next few hundred thousand (which I actually think it did).
 
You ride with your foot on the clutch? Does you clutch pedal have the recommended free travel. Either can cause wear on the thrust surface of the main bearing.
 
Thanks for all the responses! I do not think I ride the clutch but it is used often. Both sides of the thrust bearing have the same visible damage. I will check clutch pedal free travel. The rods do not show any discoloration but I am inspecting everything laying on my back looking up into crank so if a rod is slightly twisted I would not be able to tell.
Thanks Again!
 
When you install the main thrust bearing you need to pry the crank one direction and pry the main cap the other way. To line up the thrust surfaces if not the surface of the bearing in the block and the one in the cap will be staggered and the there won't be enough crank end play.
 
Bob, this just looks like thrust wear to me. If you trust your gage and the pressure was indeed coming in and out, you ve got a problem somewhere that you have yet to discover. You said that you removed the oil pump. Have you inspected it and the gasket to make sure it couldn t be the problem? It just sounds to me that something got loose enough to impede and at the same time that loosening compromised the high pressure side. Did you take a good look at what might be laying in the oil pan?

A knock without a miss that develops instantly is almost always a slipped or spun rod bearing which comes with and sometimes caused by a loss of oil pressure. You say you don t have this and I can t think of any other possible immediate knock except some it coming loose in or around that oil pump. M

Good luck with it,
Joel
 
Joel there was nothing out of ordinary in oil pan. I had the original oil pump parts replaced when I did the rebuild just a few years ago but the cost of a kit it nearly the cost of a new pump so I'm going the new pump route. I'm also getting a new oil pressure gauge to insure myself that my original gauge still works. There were no spun bearings and all bearings were wet with oil. Hopefully this will solve my problem. Thanks for your help.
 

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