High Performance Engines Bearing Clearance

Careful measurement is the key to quality work. Work slowly when measuring rod journal diameter to guarantee an accurate reading by ensuring the micrometer is perpendicular to the journal surface.>>>

If you're one of those guys who take pride in building a high-performance engine, you know that one of the most important aspects of making that rotating mass live a long and powerful life is setting bearing clearances. With more people building killer street engines, we thought it would be a good idea to touch on a few aspects of main- and rod-bearing clearance, how to accurately measure housing bores and crank journals, and easy solutions for too-loose or too-tight clearances.

How To Measure Bearing Clearance
While Plastigage has its supporters, for this exercise we prefer to use a micrometer and a dial bore gauge to accurately measure journals and inside diameters to determine bearing clearance. The most important thing to remember is to always be consistent and measure everything using the same tools the same way. This also applies to temperature. Crankshafts, rods, bearings, and micrometers are all constructed out of varying grades of steel or iron that expands and contracts with temperature. All quality micrometers are designed to be used at a standardized temperature, usually 20 degrees C or 68 degrees F. Since hot parts expand and cold parts contract, it's easy to see how temperature can affect both the steel micrometer and the part you are measuring. even a temperature difference of 10 degrees on either side of this standard can affect accuracy. We did a quick test for rod-bearing clearance between 58 degrees and 72 degrees and discovered that as the tools and parts got warmer, our bearing clearance tightened by 0.0005 inch! Imagine what adifference of 30 degrees (70 to 100 degrees F) would be worth.

We set up the dial bore gauge by using the same micrometer to set a 2.1000-inch diameter. Some engine builders set the dial bore gauge at the rod journal size and read the actual clearance on the dial indicator. We prefer to set the dial bore gauge at a common journal diameter and do the math. This way, we measure all eight rod bearings' inside diameters with one setting.>>>

We're using a set of Fowler outside micrometers from Precision Measurement Supply along with a dial bore gauge. If you've never used a micrometer, just click on precisionmeasure.com/microm1.htm and this site will run you through the procedure. For our small-block Chevy, we measured a typical rod journal using a Fowler 2- to 3-inch outside micrometer. First, measure the outside diameter of the journal. our rod journal measured 2.0989 inches, which we wrote down. now, using that same micrometer, we set up our dial bore gauge to read 2.1000 inches. Most engine builders skip the step of measuring the housing bore, but it's worth mentioning that most bearing-clearance issues stem from inconsistent housing-bore diameters. All quality rod and main bearings are very close in terms of consistency, but a tolerance stack of a tight housingbore diameter combined with a slightly oversize rod journal can contribute to tighter than acceptable rod-bearing clearance.