Main Bearings and Crankshaft.

TM 5-4320-258-34

Rotate the crankshaft and roll the bearing shell from the

(10) Check the crank pin bearing journal-

cylinder block.

to-connecting rod bearing clearance with plastigage.

Lay a piece of plastigage material on the crankshaft

journal and install the connecting rod bearing cap.

Torque the nuts to 35 to 40 foot-pounds. Remove the

bearing cap and compare the width of the flattened

plastigage material with the scale markings on the

plastigage package to determine the clearance. The

bearing-to-journal clearance shall be 0.0006 to 0.0022

inch. If clearance is beyond these limits, replace the

bearing and/or the crankshaft as required.

(11) As an alternate method of checking

crank pin bearing journal-to-connecting rod bearing

clearance, install a 1/2-inch piece of 0.0022-inch-thick

feeler stock between the bearing and journal, and install

the bearing cap. Tighten the connecting rod cap bolts to

35 to 40 foot-pounds torque. Rotate the crankshaft to

detect drag. If clearance is within tolerance, a definite

drag will be felt. Disassemble the rod cap and remove

the shim stock. If clearance is not within tolerance,

replace the connecting rod bearings and recheck the

clearance. If clearance is still not within tolerance,

replace the crankshaft.

(12) Lubricate the crank pin bearing

Figure 4-16. Removing upper bearing shell with angular

journals and the sleeve bearings with engine oil. Install

pin.

the cap (13, fig. 4-14) on its connecting rod (20) and

crank pin bearing journal; secure with the two bolts (12)

(7) Pull the oil thrower (21, fig. 4-14) and

and nuts (11). Tighten the nuts to 35 to 40 foot-pounds

gear (22) from the crankshaft (40). Remove the key

torque. Install the cotter pins (10).

(23) and remove the thrust plate (24).

(13) Secure the remaining connecting rods

b. Cleaning and Inspection.

to !the crank pin bearing journals.

(1) Clean all parts with cleaning solvent

(14) Install the engine oil pan (para 4-6c).

(FED. Spec. P-D-680). Clean oil passages in the

(15) Install the cylinder head (para 4-3 c).

crankshaft with a rifle cleaning brush. Make sure all

4-11. Main Bearings and Crankshaft.

passages are open.

a. Removal and Disassembly.

With the

(2) Inspect the crankshaft for cracks, worn

engine mounted on an engine overhaul stand, proceed

or scored journals, damaged threads, and burred

as follows:

keyways. Refer to table 4-1 for wear limits. If magnetic

(1) Remove the cylinder head (para 4-3 a).

particle inspection equipment is available, use it to

(2) Remove the oil pan (para 4-6 a) and oil

check the crankshaft for hidden flaws. Replace a

pump (para 4-7 a).

damaged crankshaft.

(3) Remove the gear cover (para 4-8a).

(3) Inspect the gear for cracked, chipped

(4) Remove the flywheel and flywheel

and broken teeth; replace a damaged gear.

housing (para 4-9 a).

(4) Inspect the bearing shells for cracks

(5) Remove the pistons and connecting

and scoring. New bearings are smooth and highly

rods (para 4-10 a).

polished. After a few hours of operation, the bearing

(6) Remove the bolts (26 and 30, fig. 4-14)

surfaces become leaden grey in color and develop

and flat washers (31 ) that secure the main bearing caps

minute craters which give the bearing surfaces an

(27, 32, 34, and 37) to the cylinder block. Loosen the

almost cellular appearance.

This is a natural

bearing caps by tapping them with a plastic hammer.

characteristic of the bearing and does not indicate

Remove the bearing caps and lower bearings (29, 33,

bearing failure. Replace bearings if they are scored or

36, and 39).

damaged. Check the bearing thickness with a ball

micrometer. Check several locations on the bearing. If

Note: Upper main bearing shells can be

thickness is less than 0.920 inch, replace the bearing.

removed without removing the crankshaft. To remove

(5) Inspect the bearing caps for cracks and

the upper shell, remove the main bearing cap at the

distortion and for burrs and gouges of the seating

defective bearing and remove the lower bearing shell.

surfaces. Clean up any burrs with a fine stone to assure

Insert a pin with an angular head in the oil hole of the

proper seating of the bearing cap on the block.

crankshaft as shown in figure 4-16.

4-22