Skip to main content
Skip to page content

Engine assembly: The bottom end

In our previous segment on engine assembly, 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 engine 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. 

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 that 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 on 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 our way backward, beginning with main bearing number one (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 ensures 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.

Check the clearances
Using an inside micrometer or dial bore gauge, measure the bearing inside diameter. Much like the crankshaft, we tend to measure the bearings (within the bore) in several different locations. Subtract the crankshaft outside diameter (measured previously) for journal number one from the bearing bore diameter. That resulting figure is the bearing clearance. Check the figure against manufacturer specifications. If the bearings are out of spec, you’ll have to juggle bearing halves (you can buy slightly under- and oversize bearings for popular engines) to come up with the appropriate clearance figures.
 
Repeat the entire process for all of the main bearings and caps. Once complete, remove all of the caps. Keep each cap and bearing intact. Leave the lower bearings in the cylinder block.
 
Installing the crankshaft
Depending upon the engine you have, it can be equipped with either a one-piece or two-piece rear main seal. No matter what format, it must be installed next. In either case, install the seal so that the lip faces inward (toward the engine). Clean the seal groove with a shot of brake cleaner and a fresh shop towel. The groove must be clean and oil free for the sealant to work properly. Apply a small amount of RTV silicone sealer on the seal groove in both the cylinder block and the main cap. Wipe up any excess (a wet finger works perfectly). Install the bottom half of the seal, or in the case of a one-piece seal, gently tap into place over the crankshaft (you can use a seal driver, but most seals easily tap on).
 
Apply motor oil (the same SAE 30 oil works) to the main bearings. Alternatively, you can use engine assembly lube (shown in the photo). It sticks with more tenacity than oil, providing more protection during the initial startup. Apply a small amount of engine oil or assembly lube to the main seal lip. Lower the crankshaft into place.
 
Reinstall the number one cap and the thrust bearing cap only. Seat the caps (using a soft face hammer). Install the bolts by hand, but don’t tighten.
 
Checking thrust clearance
Using a soft face hammer (plastic dead blow or brass), tap the crank nose (moving the crankshaft rearward). Install a dial indicator to read on the crank flange or nose of the engine. Using a large (clean) screwdriver or pry bar, move the crankshaft backward. Zero the gauge on the dial indicator. Pry the crankshaft forward and check the reading. Record the measurement. Next, torque the caps to specs and repeat the process. Compare the measurements. If the second reading is less than the first, there’s a chance the rear main cap shifted and the thrust surfaces are misaligned. Shift the cap and recheck. By the way, this doesn’t regularly occur, but if it does, you might have to check and shift the rear main cap a couple of times to get it right. Compare your final thrust clearance figure to the manufacturer specifications. Finally, install the balance of the caps (and bearings) and torque to specifications.
 
Checking the rod clearances
Beginning at the front of the engine, use your micrometer to check the overall diameter of each of the connecting rod journals on the crankshaft. The process is exactly the same as we used to check the main bearing clearances. Check each journal in multiple locations and record those figures.
 
You can now check the rod bearing clearances. Use the same process we used for the main bearing caps: Install the bearings with the tabs aligned. You can match the numbers on the rods or check to ensure the chamfers are all on the same side and install the caps. The bearings (and caps) are aligned tang to tang (not offset). Using engine oil as the lubricant, torque the cap bolts to the recommended figure and measure the bearing ID with a bore gauge.
 
Subtract the journal diameter from each of those figures to determine the clearance. Repeat the process for all of the connecting rods. Like the case with the main bearings, if the clearances are out of spec, you can often correct the dimensions by juggling bearing inserts.
 
Once all of the rod clearances have been checked, you can loosen the rod bearing cap hardware, but don’t completely disassemble. You need to keep the rods and their respective bearings in order. Wrap the engine in the plastic storage bag. You’re done with this segment.
 
In the next part of our series, we’ll show you how to file-fit piston rings to each cylinder, how to assemble rods onto pistons, and, ultimately, how to install all of the pistons and rods in the engine short block.

Tools used for this part of the assembly:
  • Dial bore gauge
  • Dial indicator and magnetic base
  • Two-third-inch micrometer
  • Half-inch drive torque wrench
  • Three-quarter-inch and half-inch drive sockets
  • Seven-sixteenths-inch 12-point half-inch driver socket
  • Large pry bar
  • Deadblow hammer
¥ Deadblow hammer

Using “Plastigage”

If you don’t have access to a dial bore gauge (or inside micrometer set) or an appropriate outside micrometer, you can still check bearing clearances with “Plastigage.” Plastigage is a special extruded plastic thread (think of an advanced version of kid’s plastic modeling clay or Silly Putty). The difference though is the stuff is engineered so that it includes controlled crush properties. It’s available at most auto parts stores, in various different crush dimensions to coincide with the clearance figures for your engine.
 
To use it, loosen the bolts of bearing cap number one. Remove the bolts along with the cap (and bearing). Wipe all traces of oil from the crankshaft and bearing surfaces (we use a paper towel).
 
Next, tear off a short piece of Plastigage (it’s sold in a long, thin paper envelope). Place a section of Plastigage on the center of the crankshaft journal, oriented front to back or diagonally.
 
Install the bearing cap and bolts. Torque to specifications, then loosen the bolts once more and remove the cap. You’ll find the Plastigage has crushed on the crankshaft journal.
 
Using the envelope the Plastigage was packaged in, you’ll find a scale on one end. Compare the scale to the crushed Plastigage on the bearing journal. This is the clearance figure. If the clearances are within specifications, you can move forward: Clean the journal (it wipes off with a towel soaked in brake cleaner) and repeat the process for all bearing journals. You can also use the same format for checking connecting rod bearing clearances.

Alt Text

The first step in determining bearing clearances is to measure the crankshaft main journal diameter. Here we’re using an outside micrometer to get the measurement. The article text offers details on how it’s done and tips on using a mic. Check all crankshaft main bearings and record your figures.


Alt Text

Next, install the main bearing insert for journal number one in the cylinder block. Note the orientation. The oil hole in the bearing half aligns with the oil hole in the main bearing. 



Alt Text

The matching bearing insert for the main cap is installed next. Because of the need for bearing crush, the insert will seem marginally too big, and a minute portion of the insert will protrude past the edge of the cap (the same applies to the insert in the block).


Alt Text

Install the cap and torque the main bearing cap to specs. Typically, we begin with the inside bolts and work outward. In addition, it’s best to use three steps on each of the fasteners in order to “sneak up” on the final torque figure.


Alt Text

With the bearing installed in the cap and the fasteners torqued to specifications, you can measure the main bearing inside diameter. Here, we’re using a B&B Performance dial bore gauge for the measurement. Subtracting the crankshaft journal figure from this measurement provides the bearing clearance. Record all measurements and repeat this process for all bearings except the rear main.


Alt Text

Before measuring the rear main bearing inside diameter, install the oil pump and torque to specifications. The reason is there is always the chance for some distortion of the cap once a heavy oil pump is installed. Here, a huge Titan G-Rotor oil pump is installed and torqued to specs.


Alt Text

Once you’ve checked all of the main clearances (and they’re all within specification), you can install the crankshaft. You can use conventional engine oil for the installation or assembly lube. Apply a light coating to the lower main bearing halves.


Alt Text

Next, the rear main seal is installed. With a one-piece seal such as this, it must be slipped over the rear of the crank prior to lowering the crankshaft into the block. The article text offers more detailed info, but in this photo you can see the installed seal.


Alt Text

With all of the lower bearing inserts in place, you can gently lower the crank into the main bearing saddles. You can reinstall the rear main bearing and cap along with the first (front) bearing cap.


Alt Text
Checking the crankshaft thrust dimension is next. You’ll need a dial indicator setup to read front-to-rear movement on the crankshaft. Depending on how you choose to perform the job, the dial indicator can be set up on the nose of the crank or on the rear flange. For this job, we have access to the flange on the engine stand (it needs to be tapped with a hammer), so we set the dial indicator up on the nose. The text offers details on how this job is accomplished.

Alt Text

All of the main bearing cap bolts (or studs) can be torqued in place. We use a half-inch drive torque wrench for the job, and while we have three other torque wrenches in our tool collection, this long handle one makes torquing large numbers easy. What’s with the blue line on the outer row of bolts? It’s used to note which caps have been fully torqued. That way you can’t forget if you’re called away. We always double-check them.


Alt Text

Measuring the rod bearing clearances is next. Here, the number one and two rod journals on the crankshaft are measured using a micrometer. Much like the main journal measurements, it’s a good idea to check in multiple places around the journal. This determines if the journal is in fact round. Repeat the process on all of the bearing journals and record the measurements.


Alt Text

At this point, the connecting rod bearings can be installed. Note the chamfer on the bearing is designed to match the chamfer on the connecting rod. You’ll also note the bearing tangs are next to one another. If the connecting rod (and cap) is unmarked, this orientation ensures the cap is installed correctly. When assembling the engine, the chamfer faces the large fillet radius on the crankshaft.


Alt Text

With the bearing installed and the cap correctly oriented, you can torque the connecting rod fasteners to specification. In this case, the GM Performance Parts connecting rods mandated 30W oil as the lubricant during tightening. Sneak up on the final figure just as you did with the main bearing caps.


Alt Text

Here, we’re using our B&B Performance dial bore gauge to determine the inside diameter of the rod bearing (installed within the connecting rod). Once this figure is determined for each connecting rod, subtract the crankshaft rod journal dimension to determine the oil clearance. FYI, the best way to measure bores such as this is by way of a dial bore gauge.


Read the next article in this engine assembly series.