Why the focus on bearing wear?

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Jul 11, 2003
In many of the discussion here low bearing wear as indicated by low lead readings in UOAs is considered the holy grail. However, I submit that bearing wear is not usually the life time limiting factor in the automotive engines I've seen.

Yesterday I was visiting with one of my car buddies and had a look at the Ford straight 4 motor he is rebuilding in his shop. The trigger for the rebuild was low compression and high blow by. Tear down indicated worn rings and marginal valve seats.

This got me to thinking that in most cases of engine tear down this is what I've seen as well. I do this kind of thing as a hobby, not as a profession so my sample is less than 10 motors, but bearing wear out or failure has never been the thing which caused these engines to need rebuilding. Valve train wear and cylinder/piston wear have always been the trigger points. Generally one installs new bearings as a matter of course.

However, it seems that for most engines we should be concerned first about the metals used in the valve train, rings, pistons and bores and only secondarily about the bearing metals. These are surfaces which are generally not pressure lubricated.

For most engines this would mean that iron and aluminum would be the ones to watch most closely.


I agree, except that the "bearing wear" referred to may in fact be cam bearings, and the iron may originate from the cam surfaces or the valve train itself, don't know really, but it may be as good a gauge as is possible with a UOA. I would like to see a little more emphasis put on a couple of other factors as well...like the amount of VII, since I have recently heard of many "new design" engines succumbing to stuck rings. And, more recently, the GM6094m spec. Not that I feel it is very important, just that I looked at several(ALL) brands of oil on Wally's shelves the other day and not one of them even mentioned the spec. This last one is a personal question since my '04 5.3L truck calls for that spec.

Valve train wear and cylinder/piston wear have always been the trigger points

Good point.
Many good points made by all above. My recent revelation and closer look at what was perceived as main/rod bearing wear numbers was due to the fact several people kept using lead levels as "proof" that the oil they espoused was superior to all other oils.

In the case of Ford engines, I actually read a quite lengthy article about many of the features of the Ford modular engines to include the use of aluminum rod/main bearings...but I read that article 14 years ago!! Maybe when you guys get a few more miles/years on your brain you'll understand why it takes a few of us a little longer to connect the dots when we're left alone in the corner.

You long time members also have to keep in mind that newer members may not have seen any of the threads where you discussed this subject at length in the past. Further, sometimes I feel it's more diplomatic to just mention bearings composition so that it might expand the horizons of thought amoung newer members as opposed to waiting to bounce on someone less knowledgeable. Not that any of you do this, but I have seen it more than a few times.
I focus on Fe and Cr/Al wear from the valvetrain, cylinder walls and rings/pistons. You want to look at the engine parts that function under very high pressures and boundary lube conditions ....

Bearing wear should NEVER been an issue with clean oil of the correct viscosity ....


Originally posted by jthorner:
Generally one installs new bearings as a matter of course.

Fully valid for BMW where one of the main issue is piston rings wear or aged valve seals. Cylinder walls are usually still look very good even after 250-300K km. The valve train may already have the first signs of wear, but it's often caused by some drivers' love to lugging. Bearings could make another 100-200K km and usually to be changed just to avoid their "turning" after new piston rings are installed.
The most isidious problem for internal combustion engines is deposit control. Ash wears valves, sludge and varnish destroy seals and cause localized hot spots. Sludge can also block oil flow witch will increase wear.

I have seen deposit control as a key to long engine life since I was a young tech. If you oil is leaveing a lot of itself behind the best wear numbers in world will not save you.

The reason so many people point out lead is because it should never be an issue if you are useing the proper viscosity oil. We see a lot of bearing wear though on this site and very few people on this site are useing a thick oil. If your oil and lube system are functioning right your lead chould be single digits with occasion low teen number. Lead should never exceed other wear metals especialy iron but we see a lot of this on this site. Iron wear is to be expected from cams,valves and rings. If we see cromium spike and tin then we know that their are issues.

P.S. The above general UOA guide lines are useing 7500 miles or 6 months as the OCI.
You are right that the best signal is iron, as it shows the sleeve or block wear and cam wear. Aluminum is important but you have to subtract 1/3 of the silicon value unless you know the virgin oil had silicon in it or it is coming from seals.
Bearing wear shows viscosity breakdown, shear, or low oil pressure. It is a big issue here with some oils that should not be on the market.
Copper is quite variable depending not only on the oil, but the driver and driving habits. Those of us who race, shift quickly, drive hard, etc. will have higher copper numbers from the thrust bearings that only come in severe contact during those activities. The disign of the thrust bearing also varies from engine to engine, with some being much more susceptable to wear.
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