main bearing trouble

[Al Bass]

a week ago my engine started knocking about 12 miles from home so I parked it and just got around to pulling the pan and the center main was the culprit. the engine has 4500 miles on a complete rebuild, the crank was turned 20 to 30 over. now I'm thinking of replacing the crank with a new counter balanced one thinking the old crank was the problem.

[Kurt in NJ]

I doubt the crank is the problem.

Most likely it was a temperature problem when the cap was poured ---tinned surface too cold, looks like the babbitt didn't bond.

I had that problem when I first went to check the clearance at 3000 miles after a professional rebuild from a place recomended by a local restorer, later I got to look in 2 other engines done by the same place before they were started ---one still had wadding in the oil passages, the other had no oil grooves ---decided to learn how to do it myself.

[Tom Wesenberg]

Now would be a good time to install a counterweighted crank. The counterweights take a lot of force off the babbbit.

[James Rogers]

Kurt is correct, the babbitt didn't bond. When this happens the babbitt is like an insert bearing and will not be able to withstand the pounding. It also looks like the babbitt has some lead making it too hard and brittle. I see this problem all the time. I make sure all my caps (front and center) are at 350 degrees before pouring so the babbitt bonds correctly. This is the only way these 2 caps have to hold the babbitt in place. Chevy used babbitt shells in this period but have 1/2" holes in the cap to stake the shell in place and they work very well. Model A's don't have any holes to stake the shell so, the bearing gets slammed to pieces.
Tom is also correct, now is the time to install a new counterweighted crank, either the Burlington crank or one with weld on weights. I use and carry both.

[Kohnke Rebabbitting]

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Originally Posted by Al Bass

a week ago my engine started knocking about 12 miles from home so I parked it and just got around to pulling the pan and the center main was the culprit. the engine has 4500 miles on a complete rebuild, the crank was turned 20 to 30 over. now I'm thinking of replacing the crank with a new counter balanced one thinking the old crank was the problem.

Kurt, the reason the babbitt didn't stick to the tinning on this cap, is because the cap is void of any tinning what so ever, it has never had any. The correct tinning temperature on any shell to be tinned, should be 610 to 640 ONLY, above, or below, will give less then satisfactory results. You can pour Tin Base at 750, no lower, we have our Pots set at 950, to a 1000 degrees, as soon an the ladle ful comes out of the pot, the Temperature drops as fast as a pencil dropping on the floor. Lead will NOT make Tin babbitt harder, lead is soft, it makes Tin babbitt soft, and will melt at a lower Temperature, watering down a good Tin Base. You have to have a Tinned shell at least 610 to 640 to pour, and the Jig to at least 850, we go to 950, to a 1000. The last thing, comparing a 1928 Chevy, to a 1928 Model A, there is no comparison, as you have two entirely different main bearing designs that work equality well. Herm.