CLEARANCES VS. CAVITATION EROSION
Cavitation erosion of a bearing occurs when rapid movement of the shaft away from the bearing surface causes vapor bubbles to form in the oil film. When these bubbles break, the resulting force causes erosion of the bearing soft overlay layer. Appearance and location of cavitation erosion will differ with operating conditions due to varying load patterns in different engine applications.
According to Havel, the nickel dam in the H-series and copper-indium intermetallic compound in the V-series helps to resist further penetration. Prolonged exposure will eventually result in erosion of the nickel or copper-indium dam. H-series bearings feature a thicker nickel dam to resist cavitation longer. Eventual penetration of the nickel dam causes copper particles to break loose, enter the clearance gap and become embedded into the bearing surface.
So far, the most effective means of controlling cavitation erosion seems to be a reduction in bearing clearance. This has worked in IRL and NASCAR applications. As an example, Aurora IRL engines running 0.0028″ rod clearance experienced cavitation erosion, but those running a slightly decreased clearance of 0.0020″ showed little or no cavitation erosion.
In order to gain further insight from a race engine builder’s perspective, we spoke with three noted builders. Following are their comments.
We’ve tested cryogenic treatments on main and rod bearings but, while cryo treatment has its place in other areas, they haven’t seen a benefit in terms of bearings. The new coating for the TriArmor bearings is far superior to the original coating and holds up very well. The new coatings have improved a ton. Oil clearances have definitely become tighter, with 0.0011-0.0015″ now becoming commonplace, especially due to smaller journal sizes and the use of synthetic oils (depending on the application, they’re running 0-10 up to 20-50 weights, with the majority using 10-30 and 10-40).
Depending on the application, our engines are running less oil pressure, and less parasitic drag (we’ve taken full advantage of oil shedding coatings to help reduce drag). The only secret I’ll share is our method of installing rod bearings. Instead of finger-pressing them into place, we roll the bearing shells into place in a back and forth motion for improved seating uniformity, which removes any high spots.
Power Source Racing Engines
Fox Lake, IL
Unlike a few years ago, there are actually quite a few similarities between street high-performance engines and race engines in terms of bearing applications. Race engine bearings used to run gobs of clearance, upwards of 0.003″ or so, based largely on the oils we were using. Today, with thinner synthetics and improved oiling systems, we’re able to run tighter clearances in the neighborhood of 0.0015″ on rod bearings, and sometimes less. Today we’re able to do this while still providing better bearing life and decreasing friction at the same time. Granted, we pre-heat our oil before the engine is started in order to get optimum flow from the start, so it’s a more controlled environment as opposed to what a street engine will see.
In an effort to save weight, our main journals are down to 2″. Our main bearings clearances get down to almost as tight as the rod bearing clearances. We ball-mic each and every bearing to verify thickness and we even check for straightness, allowing us to categorize bearings. However, the Clevite bearings are so precisely made that we can pretty much just run them out of the box. Once in a while, we do mix and match bearings to achieve desired clearances, but we prefer not to.
Sometimes we’ll run a half-under on one side and a standard on the other side. Clevite TriArmor bearings are already coated, but not on the thrust faces. That’s good, since we sometimes lap the thrust faces on a granite block in order to fine-tune out thrust clearance.
As far as crank oil holes are concerned, we simply deburr the holes to break off the edge. Years ago, we used to radius-sweep the holes, but you get too much bleed-off doing that, so now we simply deburr the holes, removing as little material as possible. Since Cup engines run flat tappet cams, we use a mineral oil for break-in. Once we know that everything is seated and ready to go, we then switch to a synthetic for track use. We do pay close attention to the zinc phosphate content of the break-in oil, since inferior levels of the scuff protection can cause severe problems during break-in.
Pro Motor Engineering
When we select bearing sizes, we pay attention to not only suggested clearance, but we also take into account the bearing surface from an anticipated load standpoint, as well as bearing speed, based on journal circumference.
In higher-end engines, where you plan to run smaller journals sizes, you really need to pay attention to the load-carrying capabilities.
In order to provide adequate oil delivery, we sometimes drill extra oil holes in the bearings and partial-radius grooves in the housing or saddle area of the mains to create multiple oil supply points. This is especially important in engines that use smaller bearings and will experience higher loads.
We try to run a fairly high crush while maintaining this within an acceptable range. Considering bearing load and journal and housing deflection, we want to make sure that the bearing is securely held in place. Where you have oil films that are in the tenths of thousands clearance, the bearing gets very hot. If you don’t have adequate crush, you won’t get enough heat transfer. Avoid taking housings to their maximum size to avoid inadequate heat transfer.
In many of our high-load builds, we modify the crankshaft journal oil holes in order to drive more oil to the rods. As you shrink the rod journal diameter, the load goes up. In order to get extra oil to the rod bearings, we create a slight teardrop groove to the crank main oil holes. We slightly groove the leading edge (attack side) of the oil hole. As the crankshaft rotates, this slight teardrop-shaped cavity fills with oil and is then force-pumped into the oil hole, increasing boost pressure.
This can cure problems with rod bearings that were otherwise seeing too much load. This can be done with a grinder, but we usually perform this on a CNC machine. However, you need to pay strict attention to the dimensions of the teardrop groove in terms of width, length and depth. Generally speaking, this teardrop groove is usually around 0.300″ to 0.400″ in length. If the groove is too aggressive, you could start starving the mains for oil. The specific profile of this groove controls the amount of oil pressurizing into the rod.
Understanding the specific engine’s oiling system is key. For example, in OHC engines, where hydraulics are responsible for much of the valve control, you need to maximize oiling efficiency to make sure that sufficient oil gets delivered to the top of the engine quickly.
As far as bearing clearances are concerned, for street engines that see higher loads, we tend to run somewhere around 0.003″ for mains and around 0.0025″ for rods.
We work within a window of about 0.001″ and keep a pretty tight tolerance range.
For engines that will see lots of heat for extended periods, such as endurance engines or marine engines, we tend to run tighter bearing clearances, to compensate for the fact that clearances will loosen under hot conditions.
FINE-TUNING BEARING CLEARANCE ON UNDERSIZED CRANKS
In an effort to aid engine builders in fine-tuning their bearing clearances, MAHLE Clevite recently introduced half-size H-series performance bearings. New part numbers include 0.009″, 0.011″, 0.019″ and 0.021″ rod and main bearings. These special-size bearings offer greater latitude in choosing bearings for performance engines that feature an undersize-ground crankshaft. By taking advantage of these new bearings, builders can easily achieve specific bearing clearances of +/- a half-thousandths of an inch. For example, using a pair of 0.009″ bearings will reduce bearing clearance by 0.001″ compared with normal 0.010″ bearings.
Similarly, clearance can be increased by 0.001″ by using a pair of 0.011″ bearings. It is also possible to use one 0.009″ bearing shell in combination with a regular 0.010″ shell to reduce clearance by 0.0005″, etc. These special half-size bearings are currently available for w wide range of Chevy, Ford and Chrysler applications. All bearings feature TriArmor construction and MAHLE Clevite’s unique moly graphite coating that’s distributed in a PTFE (polymer) carrier.
A WORD ABOUT COATINGS
If you’re having bearings coated, avoid applying anti-friction coating to the parting line surfaces. A 0.0003″ coating on the parting line faces can increase the total bearing crush effect by 0.0012″, which can create an initially distorted bore. As the coating extrudes from the parting line surfaces, the bearing may then lose its tight fit in the housing. Also, notes Clevite’s McKnight, coating should not be applied to main bearing thrust faces. “Since engine builders will often sand the coating off of the thrust faces in order to achieve the desired end play,” he said.