Spun Bearing: What's that?

One of the recent callers had a older car w/a spun bearing. Due to low crankcase oil. What exactly does this mean? The crankcase bearings are two “C” shaped half-shells, right? And together they form a circle that surrounds a part of the crankshaft, held in place by the connecting rod & lower cap.

So what does it mean to say the bearing is “spun”? Does that mean that circular insert isn’t staying in place w/respect to the connecting rod and cap? And instead it is spinning around as the crankshaft rotates?

If so, it seems like it would be easy to prevent that from happening, by putting ears on the “C” 's, so they can’t spin. What gives?

What it means is that the bearing surface was starved of oil, and the inner face ground into the “race” (not sure if correct term here–I mean the non-moving outer part) with sufficient force to wrench it loose from the block, where it was pressed in with some force. (All of which is bad news, obviously…)

The bearing inserts (C shaped halves) each have a tab on the outer edge at one end which fits into detents which prevent them fron turning. When a bearing insert wears down sufficiently to allow the tab to jump out of its detent it will spin over the adjacent bearing insert and become stacked and wedged and seized. Of course the flywheel will force the crankshaft to turn and the stacked, seized bearing will break loose. Occasionally the bearing will spin and seize when attempting to start the engine and the starter will be unable to overcome the the drag.
While I’m sure that main bearings can spin I have never seen one but I have seen a great many spun rod inserts and the damage is often catastrophic. Often the block and crankshaft are damaged beyond repair.

As mentioned above the bearing inserts (two halves) are clamped by the main and rod bearing caps. They are kept from moving by the clamping force and locating tabs or dowels. When a bearing spins the lubrication layer fails so the journal rubs against the insert surface. When the insert and main saddle or rod and cap becomes hot enough, their bore expands; relieveing the clamping force; and allowing the inserts to spin inside the bore of the main saddle or rod. Once that happens the insert and bore become the defacto bearing surface but without any lubrication so both begin to heat exponentially. You will know this is happening by a sudden rod knock.

I dismantled a VW 1600 engine that had spun a rod bearing. The rod would not move at all on that journal because it had clamped down on the inserts once it had cooled. The inserts were fused to the crank shaft journal and had to be chiseled off. The rod bore was serverely blued due to the heat and the bore had expanded enough to allow the rod end to bang against the case. The crankshaft probably could have been reground undersize, the rod replaced, and new bearingsinserts installed, but the case was cracked through the main bearing bore

Google ‘spun bearing’ and look at the pics. As noted above, this typically happens to a worn or otherwise troubled engine, so it’s the last step in a series of problems.

Pretty good video that explains what @“Rod Knox” is saying about the anatomy of the bearings.

If you folks haven’t heard that call, try to take the time to listen, it’s a hoot. It’s the first call of episode number 1534. About a 1975 Dodge Dart I think. The owner-lady says she forgot to add oil and that spun the bearing. So know what she did then? She decides to rebuild the engine. Herself. She takes everything apart, installs a new crankshaft, and puts it back together. She’s having a little problem with the engine continuing to run after it is turned off, dieseling, that’s why she called.

When Tom and Ray ask if she removed the head, she says “The head? What part is that?”. … lol … Later they discover she must have removed the head b/c she replaced the piston rings. She explained she had good mechanical aptitude, just didn’t know what the parts were called. So she had to go to the parts store with the part in hand, point at it, and ask for a new one. I like that lady. I definitely like the way that lady thinks.