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Engine Assembly, Part 2: The Bottom End
By Wayne Scraba/
Printable version
Degree of DifficultyModerate
Estimated Time240 minutes
240 minutes
Measuring Clearances, Checking Dimensions and Installing the Crankshaft -
In the last (and first) segment, "Engine Assembly, Part 1: The Cleanup," we showed how to (fastidiously) clean various engine components, and how to prep the cylinder block for assembly. Here we’ll get into the actual meat and potatoes of assembly: measuring main bearing clearances, measuring rod bearing clearances and checking crankshaft thrust dimensions. In the process, the crankshaft will be installed, the main bearing caps will be torqued and the bottom end will be readied for reciprocating component (rod, wrist pin and piston) installation.
Engine Assembly Includes Readying the Bottom End
Sounds complex, but the truth is there’s nothing fancy here except persnickety measurements, attention to cleanliness and plenty of patience. Bottom line: If you can muster up the persistence for detail, you can handle the job. It’s that simple. Here’s how it’s done:
Measuring the Mains
The first step is to measure the crankshaft main bearing journal diameters. This is best accomplished on your workbench. We use a micrometer to check the dimensions in an “around the clock” pattern on each journal. What that means is, we check each of the main journals in multiple locations. To properly use a micrometer, slowly tighten the spin wheel until the mic contact points meet the crank journal. Spin the bottom thumbwheel (ratchet stop) until you feel three clicks on the micrometer as it contacts the journal (keeping in mind you don’t want to scratch the journal surface either). Double check to ensure that the mic contact points are touching the journal evenly (not cocked to one side). Lock the micrometer and check the reading. Record each reading as you go around the journal. In essence, you’re accomplishing two things: You’re checking for crankshaft main journal out of round (if the readings from different points differ) and you’re also checking the outside diameter of the crankshaft journal.
Follow the same steps around each main bearing journal and record each set of figures for each journal. On a typical modern V8, you’ll have five bearing journals to measure on the crankshaft.
There are two different times you can measure rod journal dimensions: right after you’ve finished checking the mains or later once the crank is installed in the engine. If you’re confident the crank was properly machined, you can save those steps for later (which is what we’re doing here). If you’re not so confident regarding the crank accuracy, it’s best to measure it now. That way, if the dimensions are off you don’t have to go any further on the engine assembly job.
Finally, when using a micrometer, keep in mind that heat has an effect upon readings. Never carry a mic around in your pocket and don’t hold it for excessive amounts of time. Additionally, when storing the micrometer, be sure the measuring contact points are left open so that temperature variations do not stress the instrument.
Next up, each main bearing has to be installed, and the assembled diameter has to be checked. The bearings should be cleaned and dry. We start at the front and work out way backward, beginning with main bearing number 1 (bearing caps are usually numbered and marked with an arrow facing forward). The idea here is to install the bearing, torque the bearing cap and measure the inside diameter of the bearing bore with the bearing installed. More detail below:
Install the Main Bearings
To install the main bearing, you’ll note there are tangs on the bearing insert (in the old days, they were sometimes called bearing “shells”). Most engines also have an oil hole in the block that coincides with the upper bearing insert. This oil hole links the main bearing to the main bearing supply machined within the cylinder block. Only one half of the bearing set (inserts) per main journal cap will be equipped with an oil hole. It’s essential you get them right (hole in the bearing coinciding with the hole in the cylinder block machined bearing web).
Begin the installation with the tang side of the bearing insert. Install in the block and then push the opposite edge into the main bearing bore. Repeat the process in the accompanying main bearing cap. Be sure the bearing is fully seated. You’ll note there is a small amount of bearing insert extending past the main cap as well as past the cylinder block bearing bore. This is the bearing “crush.” When the main bearing cap bolts are torqued in place, the bearing “crushes” into the outside of the bore. This insures the bearing does not spin or turn during engine operation. At this time, you only need to install bearings on the number one main.
In most engines, the main bearing caps are numbered (the exception is usually the thrust bearing cap since it’s far different than any of the other bearing caps). Additionally, many main caps have an arrow that points forward: it goes toward the front of the engine. This arrangement places all of the bearing tangs on one side.
Oil the threads for the main cap bolts. We generally use good old fashioned SAE 30 conventional (non-synthetic) for this purpose. Install the front cap (with bearing inserts in place). Thread the bolts in by hand and then using a soft face hammer (dead blow plastic or brass), seat the cap against the block. Torque the cap bolts to the factory recommended specification. Generally, we use three equal steps (for example, 25, 50 and 75 foot-pounds), alternating between the bolts in each of the steps. On four bolt main caps, we start on the inner caps first then work outward. This tends to tighten the bearing cap evenly.
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