Bearing wear and oil pressure, educate me please

I have a knock that I have been trying to track down and been searching through posts about knocking. In several posts I read phrases like, "your oil pressure is within spec so your rod/crank/main bearings are ok." Why would a bad bearing cause a drop in oil pressure?

Maybe a dumb question, but I dont understand the connection a bearing set would have on the oil pressure.

Thanks.

Great question - i would like to take your question one step further, without stealing/robbing your thread....

so, part II of the above question: does anyone have a link to some serious 4.0L engine information...im talking ALL the basics...what makes it tick...how are parts interrelated....from the valves, block, bearings, oil pump/pressure, etc...etc...

Bearing wear would cause a greater "leakdown" in oil pressure, due to the larger effective "exit orifice."

Think of it like a garden hose, and you're trying to "water broom" your driveway. If you use the 3/4" hose bibb opening that's already on the end, you'll get the job done (but it takes a while.)

Same hose, but now you've screwed a "stream nozzle" onto it, with an opening of 1/4". You haven't changed the mains pressure or the volume delivery, but that pressure is not spread out as much as it was, and you have more residual pressure in the hose. You also have more effective pressure in the 1/4" stream than you do in the 3/4" stream, so you get the job done a lot quicker.

Same thing with your bearings. Nominal spec for oil clearance is .001" - .003", which allows for a thin film of oil under constant pressure to circulate around the bearing, flush it clean, and there should be enough pressure that the bearing never actually touches the crankpin. Theoretically, this would last indefinitely.

Realistically, you have "dry starts," where the engine has to get running to develop oil pressure to cushion the bearing. You also get oil contamination, which contributes to bearing wear - and, in an extreme case, substandard filters will allow particles to become lodged in the bearing shell, which may displace later, and cause a pit - which is a spot where wear will REALLY get going. Open that .001" - .003" up a couple thousandths, and you have less resistance to the supply pressure (opening up the hole at the end of the hose,) and less system pressure. Oil pressure is typically measured at/near the pump output, but will remain a constant throughout the system anyhow. Lower resistance at the bearings means lower system pressure - which affects ALL engine bearings (crank, rods, and cam) as well as lifter pressures (since they are hydraulically self-adjusting.)

Does this make sense, or have I muddied the waters some more? I'm more used to explaining this in person...

For the other question - what, specifically, would you like to know? I'm not sure that getting my book would answer the "basics" you're asking about (I wrote it with a little more experience in mind...) but it sounds like it would make a good additional chapter. I've just been working on engines in general for about 30 years now, and I'm just not sure how to organise the "basics" thoughts with the novice in mind anymore. Give me an outline, and I'd be happy to fill it!

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Engines 101: There is a gap between the bearing and the journal to allow oil to flow. When the bearings wear this gap gets larger and allows more oil to flow. When the bearings (cam, crank, rod) wear excessively, the oil pump can no longer pump enough oil to create pressure. So, when diagnosing a knock the first step is to check the oil pressure. Conclusion: when a motor is knocking and the oil pressure is low this usually points to worn bearings.:lecture:

Midnight Krawler said:

Engines 101: There is a gap between the bearing and the journal to allow oil to flow. When the bearings wear this gap gets larger and allows more oil to flow. When the bearings (cam, crank, rod) wear excessively, the oil pump can no longer pump enough oil to create pressure. So, when diagnosing a knock the first step is to check the oil pressure. Conclusion: when a motor is knocking and the oil pressure is low this usually points to worn bearings.:lecture:

Click to expand...

Half marks - depends on where you isolate the knock to (upper or lower half.)

Upper half knocks indicate lifters/valvetrain, if the hydraulic lifters are still in place (solid lifters knock no matter what.) Hydraulic lifters are self-adjusting due to hydraulic pressure provided by the oil pump. It is possible for a hydraulic lifter to "go flat" and not retain oil pressure (due to internal failure,) even though you are developing sufficient oil pressure within the engine.

Failed lifters should be replaced straight away, since they also pump oil up, via the hollow pushrod, to the top of the engine. Therefore, if you have a lifter that has "gone flat," you'll see accelerated wear on that valve due to a lack of lubrication, and you could end up with rather more work than you had originally planned. Since the head needs to come off to service AMC 242 valves, it also provides an opportunity to inspect the valvetrain more fully anyhow.

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Pretty good explainations... the actual mechanism in the water hose sweeper nozzle effect is the comparative increase in velocity of the stream and it's related increase in inertial energy as compared to the open hose end.

One possible source of engine knock that hasn't yet been discussed is piston skirt damage...generally, it is worse when the engine is cold. Another source is wristpin noise, usually heard as a double knock. It may be possible to pinpoint a damaged piston with a leakdown test depending on where the damage is and how well the rings are maintaining a seal.

Engine noises in general are difficult to describe in text and more difficult to understand... how about a thread that covers noises that includes sound bites? Beyond my personal technical ability, but possible.

bacelaw said:

Great question - i would like to take your question one step further, without stealing/robbing your thread....

so, part II of the above question: does anyone have a link to some serious 4.0L engine information...im talking ALL the basics...what makes it tick...how are parts interrelated....from the valves, block, bearings, oil pump/pressure, etc...etc...

Click to expand...

If you'd like a good introduction to how engines fuction in general, and how the different components interact, check here http://auto.howstuffworks.com/engine.htm. While this is not 4.0L specific it does have a lot of good basic information.

Pat

MudDawg said:

Pretty good explainations... the actual mechanism in the water hose sweeper nozzle effect is the comparative increase in velocity of the stream and it's related increase in inertial energy as compared to the open hose end.

One possible source of engine knock that hasn't yet been discussed is piston skirt damage...generally, it is worse when the engine is cold. Another source is wristpin noise, usually heard as a double knock. It may be possible to pinpoint a damaged piston with a leakdown test depending on where the damage is and how well the rings are maintaining a seal.

Engine noises in general are difficult to describe in text and more difficult to understand... how about a thread that covers noises that includes sound bites? Beyond my personal technical ability, but possible.

Click to expand...

I could do something like that (probably have to put it up on my site,) but I'd need donated "noises" to host. Microcassette, CD, .wav files, or maybe even cassette (I'd need to find a player somewhere.) I'd also need a description of what you think the noise is, and the conditions under which it occurs (and I'd be happy to see if I could nail it down a little further.)

Apart from some small leaks that are on my board to fix, all five of my rigs are pretty good...

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Great explanations. Those helped alot and leads me to believe that my knock is not a bearing then. I won't get specific on my knock in this thread as it seems to have some pretty good general knowledge...for me at least.

Wrist pins were mentioned above. From a general knowledge of how an engine works I'm going to assume that the wrist pin is what connects the piston head to the rod. Is that correct and how often do those go out and need replacing? Also, what would cause a wrist pin failure and would that same thing be causing other issues within the motor?

Lots of questions, thank you for answering what you have already.

Correct - the "wrist pin" ("gudgeon pin," overseas - what a lovely phrase!) is what joins the piston to the small end of the connecting rod.

The wrist pin, in our case, is press-fit into the small end of the connecting rod, and has something like .0015" of "free-play" in the piston's wrist pin bore, to allow relative motion of the piston to the wrist pin. The wrist pin should not move relative to the connecting rod (it's something like .002" "interference" - and either the small end of the rod is heated to about 800*F, or pressed in with a full ton of force.)

The piston's wrist pin bore can wear - either oversize or "egg-shaped," - which would cause wrist pin knock.

"Piston slap" usually refers to "skirt knock" - knocking of the piston skirt against the cylinder bore. Usually less of a problem than it sounds like - and fairly normal in high-mileage engines.

One of the simplest things you can do to isolate a knocking sound is to take either a mechanic's stethoscope or a length of rubber hose (with a bit of metal stuck in the end for a probe - a screw will work) and touch the probe end to various parts of the engine while holding the other end in your ear. It is possible, doing this, to isolate exactly which part is knocking, so you don't have to "shotgun" the fix.

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