Next, measure the crankshaft journal (for that specific bearing location) with a micrometer. Subtract the journal diameter from the installed bearing ID to determine bearing oil clearance. A good rule of thumb dictates that you should have about .001 inch of clearance per inch of shaft diameter. For instance, if a main journal outer diameter measures 2.375 inches, then you should have about .0024-inch clearance. In order to provide a slight margin of safety (especially for rod bearing locations), you can add an additional .0005-inch clearance.
If you thought bearings were made up of steel and some magic covering, think again. A bear
Let's say you're having a hard time obtaining the desired clearance. "Extra Clearance Bearings" (Designated by the letter "X" in the part number suffix) are available, but only for standard size journals. A word of caution: Do not attempt to increase bearing clearance by polishing the journal. Polishing is not a precision operation, and excessive polishing is likely to destroy journal geometry (straightness and roundness). If clearance is too loose, you can choose an undersize-ID bearing. According to Clevite, it is permissible to mix bearing sizes if the required mixing results in less than .001-inch difference of wall thickness. When mixing bearing sizes for select fitting, never mix parts that have more than .0005-inch difference in wall thickness. Also, always install the thickest wall bearing shell in the upper position (for rod bearing application) or the lower bearing shell position (for main bearing application).
Checking Clearances with Plastigage
Bearing clearance can also be checked with the use of Plastigage, which is a fragile, soft, and compressible plastic wire strand. This soft material is inserted between the crankshaft's journal and the bearing. Once compressed, the plastic wire retains its new crushed width and can be used as a reference to determine bearing clearance by comparing the crushed width to the gauge printed on the package.
Plastigage was designed to allow anyone-even those lacking sophisticated tools-to measure total vertical oil clearance during engine assembly. In order to check bearing clearance using Plastigage, clean the block saddles and caps, and install the upper main bearings in the block saddles and the lower main bearings in their respective caps. Install the upper bearings using prelube just as if you're building the engine.
Position the dry crankshaft carefully onto the installed upper main bearings.
Place a strand of Plastigage lengthwise onto each main journal (the strand should be positioned front-to-rear and should be parallel with the crankshaft), and simply rest the Plastigage on top of the journal.
Carefully install each main cap and tighten the main cap bolts to specified torque value. DO NOT rotate the crankshaft while the Plastigage is in place. This will smear the Plastigage, rendering it useless.
Carefully remove the main caps and measure the crushed width of the Plastigage strand, using the graduations printed on the Plastigage package envelope. Measure the entire length of the strand. Note all dimensions on a piece of paper. Proceed to the next cap and check strand width and so on. Once all main journal locations have been checked, apply prelube and install the caps, tightening to spec. Plastiguage is oil soluable so there is no reason to try removing it.
When using Plastigage to check rod-bearing clearance, install rod bearings onto the rod and rod cap. Install the rod onto the crank's rod journal (make sure rods are in correct order) with fillet chamfer sides facing journal fillets. Following the procedure we've already discussed, lay a piece of Plastigage onto the exposed rod journal, install the rod cap and tighten, then remove the cap and measure the Plastigage. Again-and we need to stress this point-do not allow the crankshaft to rotate when Plastigage is in place. The crank must remain stationary.