Analyze my bearing

[joez]

Obviously im doing the rear main, and this is how this bearing looks. Rod bearing on #8 was a bit scored, along with the crank when i replaced it ~15K miles ago. This one isnt really scored, and the crank looks great. Does this just look like cavitation or oil starvation, or is this just what 250K mile bearings look like. Im going to just put it back together and run it reguardless, just wondering what the experts think of that spot on the bearing.

 

[jarheadk5]

I see very light circumferential grooves over the whole bearing surface. In my reciprocating engines class in school, they taught us that markings like this are from minute dirt and metal particles in the oil, and are pretty normal for a bearing with a lot of time on it.

Now that wear spot along the inboard end of the bearing (the "bottom" of the bearing as you've pictured it) is another story. Again, as I was taught it's pretty common on high-time bearings, in areas where local loads are high. Normal on high-time bearings, but not too good either. If I've got myself oriented correctly with your picture, that's the rearmost crank bearing cap, so the rear cylinders are pushing the crank down into that bearing on the power stroke - mostly in that worn spot. It almost appears the base metal is showing through the babbitt coating.

If you really want to know, get some Plastigage. Put a piece in the "good" area of the bearing, and another piece in the suspect area. Do the usual Plastigage routine and see how much wear is really there.

 

[joez]

Yes, that is the rear main bearing.

High load is pretty much what i assumed was the reason for the wear, or less oil getting to that 1/2 of the bearing, causing local heat and expansion. It just seemed odd to only be in that one spot, and with no visible damage to the crank.

 

[kennyw]

It just seemed odd to only be in that one spot, and with no visible damage to the crank.

That is the purpose of the bearing in the first place. To take all the wear and tear, being cheaper and easier to replace during rebuild than the hard parts. I'd say if there is no noticeable wear on the crank the bearing did it's job well.