Cracked Main Bearing
Posted: Sun Oct 27, 2019 11:27 am
What would have caused this, and how do I prevent it in the future? (1978 2.0L Type IV GD Case)
Saddle:
Saddle:
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A) Do you have evidence of shear on the thrust surface? It looks like something happened there. That could twist the shoulder of the bearing and cause a torsional crack.
Are you referring to the thrust surface of the engine case? You can see from the photos of the pock marks in the bearing faces that something was going on there.
There are no stamped markings left on the bearing shell. The machining marks are also gone. The bearing OD surface and the case seat surfaces seem to show fretting. Could it have been the opposite situation of an undersized bearing in the case dimension getting beaten around in there due to too much clearance?
Note the abrupt change in the fretting at what I assume are the case half boundaries. Are the seats not aligned properly? This engine started life as a GEX. Could the case halves be mismatched?
Unfortunately, I don't have the required gauges to measure case bearing seat bores with the case halves assembled. However, I did use digital calipers to measure the seat diameters at the unassembled case half mating surfaces, and a micrometer to measure the bearing ODs.I also measured the crankshaft main bearing journal diameters with the micrometer and the bearing bores with the calipers (except #3, which is still captive to the crankshaft).
New discovery: The #1 bearing seat on the right case half has a bulge in it right at the mating surface. I can see it and feel it with my finger. Would this have caused the bearing to crack, or is it a result of the cracked bearing?
Get a brand new flat razor blade. Rake the vicinity of the mating surface at the bulge.