I’m speaking of cam bearings, particularly those in OHV engineapplications. What brought this to my attention was what I would call a“textbook” case of cam bearing oil bleed-out that caused the rodbearings to fail. Don’t follow me? I’m not surprised. I’m certain manyof you may be scratching your heads wondering what I mean by that.
Let’s talk cam bearings. Many cam bearings have been manufacturedwith a lining of babbitt, a soft, slippery face material. As a bearinglayer, babbitt possesses the best of all worlds for a bearing: it hasgreat embedability for particle contaminants, will tolerate minormisalignment of housing bores and is an excellent surface for marginallubrication upon start up. Babbitt will also tolerate “wear in” (forlack of a better explanation, it will push itself around to create itsown clearance).
In short, it’s the perfect material for cam bearings, especially a“light tight” full round bearing. The downside is its relatively lowstrength, and we will get into that a bit later. Optimum cam bearingclearance should be in the .002?-.003? range, but that number may go ashigh as .004? in certain applications but still be tolerable. Get toomuch above that and you will see premature cam bearing wear.
Installing semi-finished cam bearings and boring them for alignmentand clearance is a practice that is typical to most OE’s and someremans but for much of the real world and almost all rebuilder shops amore likely scenario is use of a “pre-fit” cam bearing set. Eachindividual bearing is driven into its respective bore and clearanceshould be correct. Now, we are making the assumption here that thelead-in edge of the bore in the block has proper chamfer, the bearinghas been driven straight and true and the housing bores are inalignment. Each of these factors, if not correct, will lead topremature cam bearing failure.
Another contributing factor to cam bearing failure may be the lackof surface contact on the OD. More and more blocks are cast with fullcircle oil grooves, which diminish a lot of the contact surface area onthe OD of the cam bearing. If you have a bearing that is light pressfit into a block and 30 percent of the contact area is now an opengroove for oil flow you can see how that may present a dilemma, bothfrom an installation and retention standpoint.
In an effort to account for all of these factors the cam bearing ID mayactually be at the maximum clearance or even slightly beyond. This iswhere the trouble lies. If you start out at the max bearing clearancehow long will it be until it wears beyond and causes a premature enginefailure? In particular, this is a problem with rod bearings.
As a case in point, the photos accompanying this column are from a2.2L Chevy engine. Millions of these motors have been produced and youprobably see them in your facility on a regular basis for reman. Thisengine had about 15,000 miles on it and came in with a rod knock due toall four rod bearings being worn down to the copper layer. This istypical of what you would see with an oil starvation situation (see Figure 1).
However, notice in Figure 2 that the lower rodbearing (non-load side of the bearing) is still intact with little orno bearing wear and the main bearings are in good condition as well.
While trying to determine the cause of this premature engine failureit was noticed that the cam bearings had abnormal wear on the loadedside of the bearing (see Figure 3).
Once a dial bore was installed into the cam bearing the non-worn areameasured about .005? oil clearance, while the darker shaded area inFigure 3 measured .015? clearance, an obvious oil bleed-out point. Allof the cam bearings were measuring in the same manner. So with the oilflow feeding the main bearings first, the cam bearings then fed off themains through the block and the rods, which were fed from the mainsthrough the crank.
So the rods got the least amount of oil. The combustion pressures werethen able to break down the deficient oil film of the rod bearings,resulting in the premature bearing wear. This engine was dramaticallyclose to taking out the journals completely and having a catastrophicfailure.
Upon further review of the standard bearing for this application itwas found that bearing clearance would typically exceed the optimal.However, clearance with the .002? undersize bearing that is availablefor this engine was perfect. It did not matter what bearing brand wasused; they all gave the same result.
The moral of the story is don’t take anything for granted. What mayseem to be correct under normal circumstances may not actually beoptimal, and if you build it, they will come – to your front door ifthere is a problem.
So out of all of this is don’t assume that,if your block cam bearing housing bore is on size and you haveinstalled everything correctly, the clearance is perfect because it maynot be.
Going forward there may also be other options besides the undersizeID cam bearing. As today’s engines are developed or refined, theirlimits will be pushed to develop more horsepower, torque and fuelefficiency. With these gains comes an increased load on the internalparts of the engine, not to mention the increase in temperature.Getting taxed dramatically under these conditions are the cam bearings,that vital link along the path of the oiling system.
You’ve already seen the evidence of what can happen if a cam bearingwears prematurely. Other problems related to this situation areexcessive cam lobe runout, improper timing and general poor runningconditions.
Many bearing manufacturers are now offering and moving to replacing thebabbitt-style bearing with an aluminum alloy. Babbitt load pressuretolerance is around 1,800 psi where the aluminum alloy bearing is uparound 5,000 psi. This is quite a difference, but isn’t necessarily thecure for all the ills. The aluminum alloy presents its own challengeespecially with oiling, so you will see this style bearing with moreoil feed holes, or a full groove on the ID but that is another storyall in itself.
Just because it’s new does not always mean that it will render thetolerances that you desire. As always you need to make certain thatwhat you build is right and correct because it is you who has to standbehind it. So you may want to take another look at cam bearings andtheir final ID dimensions because it might not be what you expect.Instead, it could be the root cause of failures that you neverrealized, because it was a catastrophic failure before it got back toyou.
Roy Berndt has decades of machine shop experience.He is the Program Manager for PROFormancePowertrain Products, a PER in Springfield, MO. You can reach Roy at [email protected].
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