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Measure Bearing Clearance - What's Your Problem?

Getting Clearance
Peter Constantine, via carcraft.com: I'm in the beginning stages of my first engine build-nothing fancy, just a mild 355 small-block Chevy to replace the exhausted, nonoriginal small-block in my '69 Z/28. I have a question about bearing clearances. I make my living with precision measurements so I have no problem with the actual measurements. It's the calculation I'm wondering about. Is the bearing clearance radial or diametric? For example, if a recommended clearance is 0.0025 inch, is that 0.0025 inch per side (radial) or total (diametric)? If it's total, then the actual clearance is 0.00125 inch per side. Am I correct?

Jeff Smith: Yes . . . but not always. How's that for an answer? Yes, you are correct that the distance between the bearing insert and the main journal, for example, is the total clearance, which means your above numbers are accurate with the actual clearance on one half of the bearing being 0.00125 inch with a total clearance of 0.0025. Mathematically, that is correct. Now let's take a look at what happens when we put the engine under load.

Oil moves through this clearance, creating what is called a hydrodynamic wedge or hydraulic cushion that keeps the crankshaft journal separated from the engine bearing. In the case of a rod bearing, cylinder pressure pushes down on the top bearing insert. This attempts to push the oil out from under the bearing, but oil pressure and high-pressure lubricants help maintain that wedge. More than likely, however, the clearance at that point is less than half of the total measured clearance.

Oil viscosity will have an effect on this, and a thicker oil will maintain more clearance. However, as long as the oil can maintain a clearance between the bearing and the journal, it may not be necessary to bolster this clearance with heavier oil that requires more horsepower to pump. Thinner 5W-20 and 5W-30 oils are becoming more popular since this reduced viscosity lowers parasitic pumping losses and improves power and economy.

According to a Clevite technical bulletin, the typical production engine recommendation is between 0.00075 and 0.0010 inch of bearing clearance for each 1.00-inch journal diameter. So for a small-journal small-block Chevy connecting rod of 2.00 inches, that would mean a bearing clearance of between 0.0015 and 0.002 inch. Most performance engine bearing recommendations tend to avoid clearances below 0.002 inch, even for small-journal diameters. Clearances on the tight side will increase bearing load capacity, but this also drastically reduces oil flow, which increases bearing temperature. That's why a slightly wider clearance is acceptable while sacrificing minimal load capacity.

In the case of our Olds 455 engine that uses a monster 3.00-inch main journal, clearances for this application would be acceptable well in the 0.0035-inch range. Conversely, for those Honda rod journal diameters of 1.88 used on more and more race engines, a 0.0020-inch bearing clearance would be acceptable.

More Info
Mahle Clevite
Ann Arbor, MI
mahleclevite.com

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