Oil Viscosity vs Bearing Clearance

ZeeOSix

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Per info at top of the page on the link above, this info is from the Director of Research & Development at "King Bearings".


See the section: Effect of Oil Viscosity in the first link. The effect of oil clearance on minimum oil film thickness for oils with different viscosity and at various rotation speeds is demonstrated in Figure 9. Also see the Conclusion section at bottom of the page.

It does indicate that there is a "sweet spot" of bearing clearance vs oil viscosity - clearance of 0.002-0.003" seems to work for about everything. Also, tighter bearing clearance results in a smaller MOFT, which also causes the shear rate to increase which causes the oil temperature inside the bearing to increase (3rd bullet below. and Figure 8). Most people think there is more heat rise in the bearing from thicker oil, but the shear rate and temperature rise inside the bearing is dependent on the MOFT too. Plus tighter bearings flow less oil, so that hurts the cooling too.

Conclusions
  • There is a value of clearance at which minimum oil film thickness reaches the maximum value.
  • Loose clearance results in lower temperature rise, but in greater oil leakage (risk of oil starvation), and in higher oil pressure peak (risk of material fatigue).
  • Tight clearance results in less oil leakage and lower peak oil pressure, but in greater oil temperature rise and greater sensitivity to geometric distortions of the housing and crankshaft.
  • The optimal range of clearance is 0.0015-0.003” (for a bearing with 2” diameter). Looser clearances are more suitable for highly loaded engines working at high rotation speeds and with thicker oils. Tighter clearances provide a better combination of hydrodynamic parameters in less loaded engines working at lower rotation speeds and using thinner oils.
  • Higher oil viscosity produces greater minimum oil film thickness, more power loss due to friction and more uniform oil film pressure.
  • King Engine Bearings developed a production technology - Bull’s Eye Tolerance™ - ensuring very accurate wall thicknesses.
  • The consistent shell-to-shell thickness of King bearings results in a minor difference of oil flow rate and a more stable hydrodynamic lubrication.
The fact that many engines (not in the USA anymore because of CAFE) call out a wide range of acceptable oil viscosity base on ambient temperature use goes to show that journal bearings are not going to "blow-up" on thicker oil. Instead, the real concern is to ensure the bearing design while running on the lowest viscosity called out will still survive all driving conditions while reducing the MOFT headroom to a minimum - which is driven by the main goal of getting more fuel economy. Running a little thicker oil increases that MOFT headroom before metal-to-metal contact and wear happens.

I still haven't found any technical information that says thinner oil results in more MOFT and more bearing protection. Can anyone find that? The bigger concern is using the correct "W" rating for the lowest temperature environment you are in.
 
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For bearing clearances, I aim for .001" per 1" of journal diameter. That's an old rule of thumb.

For endurance race engines, turbo/sc, and/or nitrous, I like .0012" per 1" of journal diameter.

For something like a naturally aspirated drag engine with good machine work, .0008" per 1" journal.

Select the oil viscosity according to the use and load.
 

ZeeOSix

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This is the crankshaft journal bearing and rod big end bearing clearances on a Ford Coyote 5.0L V8 from the factory service manual. Ford has recommended anything from 5W-20 to 5W-30 for street use, and 5W-50 for track use.

Note the crankshaft and rod big end journal diameters. They aren't really following 0.001" per 1" diameter rule of thumb for the crankshaft bearings ... more like 0.001" per 2" diameter. But the rod bid end are ~0.001" per 1" diameter when using the center of the specification range. If you look at the min and max specs for the rods, it could be as low as 0.0011" clearance on a 2.087" journal is along the lines of 0.001" per 1" dia like the crankshaft. Yet these engines can run anything from 5W-20 to 5W-50.

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This is the crankshaft journal bearing and rod big end bearing clearances on a Ford Coyote 5.0L V8 from the factory service manual. Ford has recommended anything from 5W-20 to 5W-30 for street use, and 5W-50 for track use.

Note the crankshaft and rod big end journal diameters. They aren't really following 0.001" per 1" diameter rule of thumb for the crankshaft bearings ... more like 0.001" per 2" diameter. But the rod bid end are ~0.001" per 1" diameter when using the center of the specification range. If you look at the min and max specs for the rods, it could be as low as 0.0011" clearance on a 2.087" journal is along the lines of 0.001" per 1" dia like the crankshaft. Yet these engines can run anything from 5W-20 to 5W-50.

View attachment 69087
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Here are 1GR-FE clearances (FJ Cruiser, 4Runner) Data source is Pure FJ Cruiser in Texas
Main Bearing 0.0007 to 0.0012 (0.0018 max)
Rod Bearing 0.0010 to 0.0012 (0.0025 max)
These clearances are less than your Coyote 5.0L specs!
Toyota recommends 0W-20 in the US, 5W-30 to 20W-50 in Europe, UAE, and Australia(?).
The 2019 Ford Mustang owners manual recommends 0W-20 to 5W-20 and 5W-30 for track use only.
Where do you find 5W-30 for street use, and 5W-50 for track use?
TrackMustangsOnline forum post: "However, the UK (and German) dealers actually refuse to honor warranty unless they run 5W-30."
 
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ZeeOSix

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The 2019 Ford Mustang owners manual recommends 0W-20 to 5W-20 and 5W-30 for track use only.
Where do you find 5W-30 for street use, and 5W-50 for track use?
TrackMustangsOnline forum post: "However, the UK (and German) dealers actually refuse to honor warranty unless they run 5W-30."
The 5W-50 was specified for the Boss 302 and for early Coyotes with the Track Pack when used for track use I believe. Roush also specifies 5W-50 if one of their superchargers are installed, be it before the car is sold, or afterwards. Same with the Ford Performance supercharger kit too I think (would have to check). The supercharged Shelby 500 also speced 5W-50, and bearing clearances are similar to the Coyote.

The Coyote has been around since 2011, and now on it's 3rd generation. I doubt the rod and crankshaft clearances have change much if at all over the years.
 
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ZeeOSix

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Déjà vu. Didn't we discuss the exact same a couple of weeks ago?
LoL, yeah but people need a reminder every Monday that engines don't "blow-up" or wear more from using thicker hot oil, but they could "blow-up" or wear more if using an oil that's too thin for the use conditions.

Bearing protection is ultimately dependant on the MOFT. Anything that causes bearing MOFT to decrease too much and causing metal-to-metal contact is something that should be avoided.
 

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^^^ I'm betting she wasn't using xW-20 in her 600+ HP race engine. :D

And his comment about oil "thick as honey" doesn't apply to hot oil. Even 0W or 5W is comparatively "thick as honey" when an engine is cold started at -25F. Yet, the bearings don't wear out or self destruct in all those cold use environments.

Look at the examples of the bearing clearances give above in this thread. Some can be 0.001" or a bit less per the lower spec range, yet the engine manufacturer specs xW-40 and xW-50 weight oils.
 
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The video stated in laymen’s terms that the tighter the bearing clearance, the thinner the oil. He also went on to mention “the more you know,” and briefly mentioned all the factors involved when it comes to bearing clearances. In a nutshell, a Toyota Camry running 0W-16 will live a perfectly long life, it doesn’t create enough “bang” in the combustion cycle.
 

ZeeOSix

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The video stated in laymen’s terms that the tighter the bearing clearance, the thinner the oil.
As shown above, a bearing with a tight clearance doesn't really have issues running a thicker oil. But bearings with larger clearance will do better on thicker oil vs thinner oil. That's why engines with 0.001" bearing clearance can specify a whole range of viscosity to use when - CAFE is not a factor.

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This is another interesting article on oil viscosity and bearing clearance. https://blog.k1technologies.com/bearing-clearance-and-oil-viscosity-explained

There are several interesting take-aways:
1) The typical rule that most engine builders follow is that bearing clearance establishes the viscosity of the oil to use within a given oil temperature range.
2) Generally, the tighter the bearing clearance, the lower the viscosity the engine can use while wider clearances require thicker oil.
3) A bearing’s load carrying capacity is directly affected by clearance. As the clearance is tightened, load carrying capacity increases.
4) Common sense dictates that bearing clearance directly affects oil flow with predicable results. Reducing the clearance will increase the restriction to flow and reduce the volume of oil past the bearing. Because of this reduced flow, the localized bearing temperature will increase. If this temperature exceeds the oil’s thermal stability limit, the oil will begin to oxidize and break down, reducing its ability to lubricate.

So maybe, just maybe the manufacturer knows what they're talking about when they recommend a 5W-20 oil.
Just because you can use 5w-20 doesn't mean you should or it's ideal for all circumstances. Driving short trips to the grocery in the winter with some highway driving at constant RPM and normal (55mph) speeds? Sure.

Driving in heavy city traffic in 90 degree weather with a lead foot, banging it off the rev limiter at every opportunity on the highway? Maybe not a good choice.
 
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So what's the consensus on heat and hydrodynamic friction when using a thicker oil in tight clearances? With reduced flow exiting the bearings and increased hydrodynamic friction from the increased viscosity, the oil's temperature rise through the bearing must be higher and bearing temps also higher.

I've noted spalling wear of the rod bearings from someone using a 20W-50 in an road racing engine with .0015-.0018" clearance. The mid-season oil change (after 5 events) showed >100 ppm of both Pb and Cu with a bunch of tin as well. The oil pan was removed, rod bearings pulled, and found spalling of the bearing surface. Oil sump temp would reach 280-290*F during the race with the bearing temps likely >350*F. Now, also note this was a '65 289ci V8 engine with stock replacement bearings. A modern engine with better alloys and aluminum block would likely take that heat better. The crank of that engine was still decent so it was just cleaned up and new bearings installed at the same clearance with a 5W-30 (Redline) used. The racing oil temp dropped down to ~260*F and they looked good as new after another 5 events.

I know Ford recommends Motorcraft 5W-50 for racing in the higher end models, but I don't know of anyone competitively racing those cars that actually uses a 5W-50 oil. The few I've had the liberty to check out were using 5W-30 or 0W-30. The drag racers are using 0W-20 or 0W-16.
 
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I've noted spalling wear of the rod bearings from someone using a 20W-50 in an road racing engine with .0015-.0018" clearance.
Same with the CAT, Cummins and others with their wet liner problem. Coolant would get so hot it would boil. When the bulbles burst it would pit the liners eventually leaking coolant into the combustion chamber. Oil can do the same causing spalling to the bearings. Wish I could find the article I read recently that talks about this. The coolant companies had to come up with a new class coolant (or additives) to aleviate the problem.
 
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Can we ALL stop with the childish "blow up" references?

The proof is on both sides of the pond.

For decades OEMs in the US have recommended low viscosity oils with overwhelmingly positive results.

In Australia, Europe etc, on the other hand, a range of viscosity has been the approach. This has worked just as well.

If both are recommended, both will work just fine. The differences in longevity are so miniscule there isn't a comprehensive study that says otherwise...that I have ever found.
 
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