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Do-It-Yourself Projects
Engine Assembly, Part 2: The Bottom End
By Wayne Scraba/
Printable version
Degree of DifficultyModerate
Estimated Time240 minutes
240 minutes
Tools Used in Part 2:
  • dial bore gauge
  • dial indicator and magnetic base
  • 2-3-inch micrometer
  • 1/2-inch drive torque wrench
  • 3/4-inch and 1/2-inch drive sockets
  • 7/16-inch 12-point 1/2-inch driver socket
  • large pry bar
  • 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 plasticine 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.
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.
Measure the Crankshaft Main Journal Diameter
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.
Install the Main Bearing Insert for Journal Number One in the Cylinder Block
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).
Next Install the Matching Bearing Insert for the Main Cap
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.
Install the Cap and Torque the Main Bearing Cap to Specs
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.
Measure the Main Bearing Inside Diameter
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.
Install the Oil Pump and Torque to Specifications
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