How to Install Main & Rod Bearings

by John Stevens J.D.

Installing crankshaft main bearings and connecting rod bearings is much more involved than simply pressing the bearings into place and bolting the components into the engine. To prevent each bearing from being destroyed or from creating an insufficient oil pressure level, a small amount of play must exist between each component and its bearing. That tolerance is measured with plastigage, which is essentially a thin plastic fiber. The plastigage compresses when the component is installed, and is then measured with an included scale. Checking and installing the bearings takes time, but will help ensure that the engine is assembled correctly.

Installing the Main Bearings

1

Thoroughly clean all oil from the crankshaft journals with a clean, dry rag. All journals must be clean and dry to ensure that a proper plastigage reading is achieved. Journals refer to the portion of a component that the component’s bearings attach to.

2

Install the bearings onto the crankshaft’s journals. Each journal uses two bearing halves. Each bearing forms a half-circle. Press a single bearing into each of the crankshaft’s journals until the bearing clicks into place.

3

Lower the crankshaft into place within the engine.

4

Cut a piece of plastigage to approximately 1 1/2 inches with scissors.

5

Place the plastigage across the bearing journal on the crankshaft, then position that bearing’s main cap over the crankshaft journal with the plastigage.

6

Torque the crankshaft’s main cap bolts to half of the cap’s total specified torque with a torque wrench. Consult the engine’s specifications manual for the precise torque setting for the main caps. For example, if the main cap bolts are to be tightened to a total of 70 ft. lbs. of torque, tighten each main cap bolt to only 35 ft. lbs. of torque. Tighten each bolt gradually, alternating between all main bolts until the torque setting is reached.

7

Tighten the crankshaft’s main cap bolts to its full torque rating. For example, if the main cap bolts are to be tighten to 70 lb. lbs. upon assembly, tighten each bolt gradually with a torque wrench until 70 ft. lbs. is reached.

8

Remove the main cap with the torque wrench and measure the width of the plastigage with the scale provided on the envelope the plastigage product is packaged with.

9

Compare the measured distance to the bearing’s clearance distance found in the engine’s specification’s manual. If the width of the plastic gauge is smaller or larger than the distance specified in the engine’s manual, the engine block, crankshaft and main caps must be taken to a machine shop for service.

10

Repeat the test for the remaining main caps.

11

Install the main caps if the plastigage measurement falls within the tolerances specified in the engine’s manual. Coat the surface off each main bearing with engine assembly lubrication, then torque to the main caps to their full torque setting with a torque wrench.

Installing the Rod Bearings

1

Thoroughly clean all oil from the connecting rod journals with a clean, dry rag. All journals must be clean and dry to ensure that a proper plastigage reading is achieved. Journals refer to the portion of a component that the component’s bearings attach to.

2

Install the bearings onto each connecting rod. Each journal uses two bearing halves. Each bearing forms a half-circle. Press each bearing into its journal until the bearing clicks into place.

3

Lower a single connecting rod into place over the crankshaft.

4

Cut a piece of plastigage to approximately 1 1/2 inches with scissors.

5

Place the plastigage across one bearing journal on the connecting rod, then slide that rod’s cap onto the bottom of the connecting rod.

6

Torque the connecting rod’s two nuts to half the cap’s specified torque with a torque wrench. Consult the engine’s specifications manual for the precise torque setting for the connecting rods. For example, if the connecting rod nuts are to be tightened to 40 ft. lbs. of torque, tighten each nut to only 20 ft. lbs. of torque. Tighten each bolt gradually, alternating between the two nuts until the torque setting is reached.

7

Tighten the connecting rod’s two nuts to their full torque rating. For example, if the connecting rod nuts are to be tighten to 40 lb. lbs. upon assembly, tighten each bolt gradually with a torque wrench until 40 ft. lbs. is reached.

8

Remove the connecting rod cap and measure the width of the plastigage with the scale provided on the envelope the plastigage product is packaged with.

9

Compare the measured distance to the bearing’s clearance distance in the engine’s specification’s manual. If the width of the plastic gauge is smaller or larger than the distance specified in the engine’s manual, the crankshaft and connecting rod must be taken to a machine shop for service.

10

Repeat the test for the remaining connecting rods.

11

Install the connecting rod caps if the plastigage measurement falls within the tolerances specified in the engine’s manual. Coat the surface off each bearing with engine assembly lubrication, then torque the connecting rods to their full toque specifications with a torque wrench.

Items you will need

References

About the Author

John Stevens has been a writer for various websites since 2008. He holds an Associate of Science in administration of justice from Riverside Community College, a Bachelor of Arts in criminal justice from California State University, San Bernardino, and a Juris Doctor from Whittier Law School. Stevens is a lawyer and licensed real-estate broker.