PISTON, RING AND ROD CLEARANCES CHART

TM 55-4920-335-14

(8) Remove six cap screws in main bearing plate at

take-off end of engine. Pry plate off and remove crankshaft

from that end of crankcase (figure 4-17).

(9) Remove cylinder blocks.

Remove all valve

tappets and withdraw camshaft from crankcase (figure 4-18).

Remove three screws and lock washers and then pry gear off

end of camshaft.

Inspection and Repair.

(1) Cylinder Blocks. If cylinders are worn more than

0.005 inch over size they should be reground and fitted with

oversize pistons and rings. Valve stems have a clearance of

0.0025 to 0.0045 inch in the valve guides. If clearance is 0.008

inch drive out valve guides and replace.

Figure 4-18. Removing Camshaft

Reassembly.

(1) Install camshaft gear on camshaft using three

screws and lock washers. The gear has offset mounting holes

to provide accurate assembly for valve timing. The gear can

only be put on the correct way for matching the timing mark

PISTON, RING AND ROD CLEARANCES CHART

PISTON TO CYLINDER

.0052 to .0062"

AT PISTON SKIRT THRUST FACES

PISTON RING COMPRESSED GAP

.025 to .035"

Figure 4-17. Removing Crankshaft

PISTON RING

Top Ring

.002 to .004'

SIDE CLEARANCE

2nd Ring

.0015 to .0035"

(2) Valves. The valve face is ground at 45 degrees

3rd, 4th Groove

.001 to .003"

to the vertical centerline of the valve stem and the valve seat

GROOVES

Oil Rings

insert should also be ground at a 45 degree angle. After

CONNECTING ROD TO CRANK PIN -

.008 to .015"

grinding, lap valves and inserts with lapping compound to

SIDE CLEARANCE

prevent improper seating. Clean valve seats and apply lapping

CONNECTING ROD SHELL BEARING

.0015 to .003

compound to valve face and put valves back into guides. Lap

TO CRANK PIN

valves by rotating them back and forth with a reciprocating

PISTON PIN - TO PISTON

.0000 to .0003"

advancing valve tool. Occasionally lift valves and reseat them

TO CONNECTING

.0002 to .0007"

in a different position to insure a uniform seat entirely around

the valve. After lapping, remove valves and wash them and

cylinder block thoroughly with gasoline or kerosene.

(3) Pistons and Connecting Rods. When replacing

shell bearings, replace a complete bearing (2 halves). The

piston skirt is cam-ground to an elliptical contour. Clearance

between piston and cylinder must be measured at the center of

the thrust face of the piston skirt. Refer to figure 4-19 for

proper clearance. The thrust faces on piston skirt are 90

degrees from axis of piston pin hole.

(4) Inspect clearances of parts as indicated in figure

4-19.

Figure 4-19. Clearances

4-11