Measuring bearing clearances is one of those steps that all good engine builders take to establish a solid and reliable rotating assembly foundation. It can be tedious trying different bearings to put the numbers where you want them, but also rewarding when you get it right.
The simple fact is that setting bearing clearances for a performance engine is something that cannot be overlooked or glossed over. There are no quick and easy ways to establish the critical clearances, regardless whether the engine is a bone-stock cruiser or a road course animal that will endure hundreds of miles of abuse.
We will run through the basics on how to measure bearing clearances and illustrate how to avoid mistakes. This will also require some critical measuring tools. Let’s just put this right out front—measuring bearing clearances cannot be accomplished with Plastigauge. Those little pieces of wax thread are not precision measurement devices and should not be used to define bearing clearances. We know this may hurt some people’s feelings, but setting bearing clearances is a process that’s too important to allow anything short of your best effort.
To begin, a few tools and precision measurement devices will be necessary. This begins with a precision micrometer sized for the range of journals you will be measuring. There are cheap micrometers out there that you should avoid. Insist on a micrometer that will measure down to 0.0001-inch. This is an absolutely necessity if you are interested in achieving accurate results.
Measuring the inside diameter of a main or rod bearing will require a dial bore gauge. The best ones are accurate down to 0.0001-inch and generally come as a set that offers a measurement range from 2 inches to 4 inches inside diameter (id). With these two tools you can quickly determine the clearances in any engine.
The process is not difficult, but does require some skill with handling and reading a micrometer. We won’t get into how to read a micrometer. If you are not sure, there are several tutorials online. It’s also important to always zero the micrometer before using it. Standards are generally supplied with a micrometer along with a tool to allow adjusting the mic for accuracy. All standards are calibrated to be used at 68 degrees F.
Before we get into the actual process, it might be good to talk about generic clearances. The commonly accepted rule that most crankshaft manufacturers prefer for street and performance engines is 0.0010-inch for every 1-inch of journal diameter. So for a 2.45-inch small-block main journal, the bearing clearance would be 0.0024-inch. For a smaller 2.100-inch rod journal, the accepted clearance would be 0.0021-inch. Factory acceptable tolerances on a stock small-block Chevy are much wider than this.
Let’s start by measuring a bearing journal. It’s best to measure a journal in at least two different planes to establish diameter and roundness. Ideally, there will be zero out-of-roundness but it’s possible to see a variation of 0.0001-inch, which may or may not be a function of measurement accuracy. Depending upon the application, new crankshaft specs call for runout and taper of no more than 0.0002-inch for both rods and mains.
Measure the crankshaft journal and record the diameter on a sheet for all the journals. For a new crankshaft, you should find the rods and mains will probably vary no more than +/- 0.0001-inch. We measured our K1 small-block crankshaft and the variation between all the rod journals was less than 0.0002-inch. Most of our rod journals, for example, measured 2.0994-inch.
With the journals measured, it’s time to set up a dial bore gauge to measure the inside diameter of the rod bearings. To begin, we set up our dial bore gauge at slightly more than 2.100-inches to establish a load on the gauge. We then set our micrometer at 2.100-inches and placed it in a protected vise to hold it secure while we set the dial bore gauge indicator up to read zero (0) at this 2.100-inch spec.
With that accomplished, we then placed a standard set of rod bearings in a connecting rod and tightened the bolts to the required rod bolt stretch figure (0.0055 to 0.0060-inch). With both bolts stretched, we then place the dial bore gauge to read the vertical oil clearance directly in line with the rod. It’s important to always measure oil clearance in the true vertical plane as all bearings are designed with an eccentricity to produce additional clearance at the bearing parting line. This is done to compensate for load because the sides of the bearing housing will pinch inward at the parting line under high load.
Our first measurement, using a standard bearing for this application indicated barely 0.0010-inch of clearance. We set our dial indicator at 2.100-inch because the Number One rod journal measured 2.0994-inch, we added that 0.0006-inch to the bearing inside diameter indicated on the dial bore gauge. This produced a true bearing clearance of 0.0016-inch, which is tighter than our minimum spec of 0.0021. This can be attributed to a tolerance stack-up issue, which is very common. This is why we measure bearing clearance.
We had a slightly larger rod journal combined with a connecting rod housing bore that we measured at slightly tighter than the middle ground spec. When clearances don’t measure properly, it is rarely the fault of the bearing insert. More often the out-of-spec clearances are due to housing bore diameters that are out of spec.
Luckily, all the performance bearing manufacturers like Federal-Mogul (Speed-Pro), Mahle-Clevite, King, and others offer bearing shells in various over- and under-sizes to allow the engine builder to customize clearances. In our case, Clevite offers a 1X bearing that adds 0.001-inch of clearance. There are several techniques you can employ to set clearances exactly where you want them.
For example, adding a full 1X bearing set would theoretically add 0.001-inch, widening the clearance to 0.0026-inch. On this engine, we decided to run the rod bearing clearances right on the 0.001-inch per inch journal spec, which can be accomplished by adding only one of the two bearing shell halves. This is an acceptable procedure as long as you never mix shell halves of more than 0.001-inch difference. So, as an example, never combine a 1X (+0.001-inch oversize) half with a -1 (0.001-inch undersize) half because the shell thicknesses will be incompatible.
When mixing shell halves, the rule is to place the thicker shell half into the loaded side of the housing bore. So in case of a main bearing, the thicker shell half would be placed in the main cap, while in a connecting rod the thicker half would be placed in the upper position with the rod. This creates a situation where, under load, the oil clearance decreases on the loaded side so the thinner shell half allows more room for the oil to enter the bearing area and maintain lubrication.
After measuring all eight rod bearing clearances and using shell halves to set the clearances, it’s very common to have a clearance spread between the rods of perhaps 0.0004- to 0.0005-inch. Let’s say our loosest rod bearing measured 0.0028-inch while the tightest rod came in at 0.0023-inch ... creating a spread of 0.0005-inch. We’ve found that switching the tightest and loosest bearings can sometimes bring the spread closer together. For our engine, we measured a spread of just 0.0003-inch.
Clearances also dictate the oil viscosity used. We’ve included a chart from Driven Racing Oil that offers viscosity recommendations based on bearing clearance and anticipated oil temperature. A street engine would operate in the 160- to 220-degree F zone unless you did some track day adventures, then the oil temperature would likely move into the over 220-degree area.
Our 400ci small-block will employ an iron block and steel connecting rods. With a rod bearing clearance of 0.0021-inch and an anticipated oil temperature range of 160- to 220-degrees F, this would put the recommended viscosity right on the edge between 5W-20 and 5W-30. Keep in mind that using a slightly wider main bearing clearance would push the proper oil choice to thicker viscosity oil to protect the wider clearance.
We won’t dive too deeply into this subject as it could easily be dozens of pages. The important point is that bearing clearance dictates the viscosity oil that the engine wants to properly protect the bearings from undue wear. There is some latitude here if you study the accompanying chart and, of course, these are merely recommendations and not hard and fast rules.
Setting bearing clearances isn’t particularly difficult as long as you work carefully and double-check to make sure all your numbers are correct. But once you’ve done that, you have just ensured that the engine has a great chance of making power over the long haul for a very long time.
|Main Bearing Clearance vs. Viscosity|
|Iron Block Engines||Oil Temperature (Fahrenheit)|
|Main Bearing Clearance||Under 160-degrees||160- to 220-degrees||Over 220-degrees|
|0.0034 - 0.0039||10W-40 / 15W-40||15W-50 / 20W-50||20W-60 / 60W|
|0.0028 - 0.0033||5W-30 / 10W-30||10W-40 / 15W-40||15W-50 / 20W-50|
|0.0022 - 0.0027||0W-20 / 5W-20||5W-30 / 10W-30||10W-40 / 15W-40|
|0.0016 - 0.0021||0W-10||0W-20 / 5W-20||5W-30 / 10W-30|
|0.0010 - 0.0015||0W-5||0W-10||0W-20 / 5W-20|
Photography by Jeff Smith