1980/01/07 Caterpillar

All Caterpillar Engines

Use Plastigage instead of 5B1161 Fuse Wire to check the clearance between the crankshaft and the main bearings and/or the crankshaft and connecting rod bearings.

If the bearing bores and the crankshaft have been correctly measured and are within specifications and new bearings are installed, it is not necessary to check bearing clearances. But, if the clearances must be checked use Plastigage.

Plastigage will not damage the bearings.

Formerly, the recommendation was to use 5B1161 Fuse Wire to check these clearances. The 5B1161 Fuse Wire is harder than the lead-tin overlay used on current bearings. Because of this difference in hardness, it is possible for this wire to damage the overlay on the bearings.

Correct Use Of Plastigage To Measure Bearing Clearances

When tight clearances [less than .004" (0.1 mm)] are checked, the readings (indications) from the Plastigage can be .0005 to .0010" (0.013 to 0.025 mm) less than the exact clearance.

For example, an exact clearance of .003" (0.076 mm) can possibly show as only .002" (0.051 mm) when checked with Plastigage.

Recommendations For Measurement Of Main Bearing Clearances With Plastigage

IMPORTANT: Plastigage must be used correctly when bearing clearances are measured.

1. Put the bearing in the cap and saddle of the cylinder block with the tabs in the slots.

2. Be sure the bore and backs of both bearing halves are clean and dry.

3. Put the Plastigage at the crown (top) of the bearing half that is in the cap. The Plastigage must not be extended over the edge of the bearing.

4. The crankshaft journal must not have any oil on it.

5. Install the bearing cap and bolts. Use the correct torque-turn specifications and procedures. Be careful not to move the main bearings out of their seat when the cap is installed.

NOTE: DO NOT use an impact wrench on the journal to be measured or a journal next to the one that is to be measured. This can cause more compression of the Plastigage which will prevent a correct indication.

6. DO NOT turn the crankshaft.

NOTE: When repairs are made to an engine in a chassis or an engine in a normal vertical position, the crankshaft must be lifted against the top of the upper bearing and held there until the clearance is measured. If the crankshaft is not lifted and held, the weight of the crankshaft will change the main bearing clearance reading. To prevent this, use a jack under the crankshaft counterweight next to the bearing which is to be measured.

If the engine is on its side in an engine stand, the crankshaft must be tight against the top of the bearing in the cylinder block saddle. This will prevent distortion of the Plastigage. When the engine is out of the chassis, put the block so the crankshaft bores are up. In this position, crankshaft support is not necessary.

7. Remove the cap, but DO NOT remove the Plastigage before it is measured. The Plastigage can be on the cap, on the bearing, or on both.

8. Carefully measure the compression of the Plastigage with the correct scale, inches or millimeters, on the Plastigage package. Write down the measurements.

The wider the compression, the smaller the clearance.

Many times, Plastigage shows a compression area with variable thickness and some twists and bends. Know the thickness specifications for the clearance and use the gauge on the Plastigage package to check if the major areas of the compression are inside the tolerances for the clearance.

With new bearings, the clearances must be near the lowest end of the specifications. The Plastigage will show smaller than exact clearances in this type of measurement.

9. Remove the Plastigage. Before the cap is installed, put oil on the crankshaft journal. Then install the cap and bearing with the correct torque-turn specification.

Recommendation For Measurement Of Rod Bearing Clearances

The procedure used to measure rod bearing clearances is nearly the same as the procedure used to measure main bearing clearances. The tips which follow are important to remember during installation of connecting rods and pistons.

1. Put plastic tubes, rubber hose or some other type of material over the rod bolts to give the crankshaft journals protection from the bolts. Also, put tape or cloth over the journals.

2. Be careful not to move the rod bearings out of their seats during installation of the connecting rods and pistons. Bearing tabs which are not in their slots can cause hot areas and the result can be failures.

3. Make sure the cap is assembled correctly on the rod.

This difference can also be caused by a twist when the cap bolts are turned to the correct torque-turn specifications.

If the Plastigage moves off the center of the journal and into the fillet area, distortion of the Plastigage is possible and the indication will not be correct.

Add a reference to this article in the August 28, 1978 Service Magazine "Change In Procedure To Check Main Bearing Clearance" on Page 2 and the August 30, 1978 Engine News article with the same title on Page 1. Also, remove from the above articles the reference to use of lead wire and give instructions to use Plastigage.

To order Plastigage or for the name of the nearest Plastigage distributor, write or call: