Measuring Main Bearing and Connecting Rod Bearing Clearance
MEASURING MAIN BEARING CLEARANCEPLASTIGAGE METHOD
Engine crankshaft bearing clearances can be determined by use of Plastigage or equivalent. The following is the recommended procedure for the use of Plastigage:
1. Remove oil film from surface to be checked. Plastigage is soluble in oil.
2. The total clearance of the main bearings can only be determined by removing the weight of the crankshaft. This can be accomplished by either of two methods:
PREFERRED METHOD
1. Shimming the bearings adjacent to the bearing to be checked in order to remove the clearance between upper bearing shell and the crankshaft. This can be accomplished by placing a minimum of 0.254 mm (.010 inch) shim (e. g. cardboard, matchbook cover, etc.) between the bearing shell and the bearing cap on the adjacent bearings and tighten bolts to 14-20 Nm (10-15 ft. lbs.).
^ When checking #1 main brg. shim #2 main brg.
^ When checking #2 main brg. shim #1 & 3 main brg.
^ When checking #3 main brg. shim #2 & 4 main brg.
^ When checking #4 main brg. shim #3 & 5 main brg.
^ When checking #5 main brg. shim #4 & #6 main brg.
^ When checking #6 main brg. shim #5 main brg.
NOTE: REMOVE ALL SHIMS BEFORE REASSEMBLING ENGINE.
ALTERNATIVE METHOD
1. With the weight of the crankshaft being supported by a jack under the counterweight adjacent to the bearing being checked.
Plastigage Placed In Lower Shell:
2. Place a piece of Plastigage across the entire width of the bearing shell in the cap approximately 6.35 mm (1/4 inch) off center and away from the oil holes. (In addition, suspect areas can be checked by placing the Plastigage in the suspect area). Torque the bearing cap bolts of the bearing being checked to the proper specifications.
Clearance Measurement:
3. Remove the bearing cap and compare the width of the flattened Plastigage with the metric scale provided on the package. Locate the band closest to the same width. This band shows the amount of clearance in thousandths of a millimeter. Differences in readings between the ends indicate the amount of taper present. Record all readings taken. Refer to Specifications. Plastigage generally is accompanied by two scales. One scale is in inches, the other is a metric scale.
4. Plastigage is available in a variety of clearance ranges. The 0.025-0.076 mm (0.001-0.003 inch) is usually the most appropriate for checking engine bearing proper specifications.
CONNECTING ROD BEARING CLEARANCE
Engine crankshaft bearing clearances can be determined by use of Plastigage or equivalent. The following is the recommended procedure for the use of Plastigage:
1. Rotate the crankshaft until the connecting rod to be checked is at the bottom of its stroke.
2. Remove oil film from surface to be checked. Plastigage is soluble in oil.
3. Place a piece of Plastigage across the entire width of the bearing shell in the bearing cap approximately 6.35 mm (1/4 inch) off center and away from the oil hole. In addition, suspect areas can be checked by placing plastigage in the suspect area.
4. Before assembling the rod cap with Plastigage in place, the crankshaft must be rotated until the connecting rod being checked starts moving toward the top of the engine. Only then should the cap be assembled and torqued to specifications. Do not rotate the crankshaft while assembling the cap or the Plastigage may be smeared, giving inaccurate results.
5. Remove the bearing cap and compare the width of the flattened Plastigage with the metric scale provided on the package. Locate the band closest to the same width. This band shows the amount of clearance in thousandths of a millimeter. Differences in readings between the ends indicate the amount of taper present. Record all readings taken. Refer to Specifications. Plastigage generally is accompanied by two scales. One scale is in inches, the other is a metric scale.
6. Plastigage is available in a variety of clearance ranges. The 0.025-0.076 mm (0.001-0.003 inch) is usually the most appropriate for checking engine bearing proper specifications.