Section III. PISTONS , RINGS, AND CONNECTING RODS

f. Install the centrifugal pump on the engine

e. Install the spark plug, crankcase breather,

(para 3-4).

and engine housing (TM 5-4320-59-12).

Section IIl.

P I S T O N S , RINGS, AND CONNECTING RODS

b. Clean all remaining parts with an approved

4-7. Description

cleaning solvent; dry thoroughly. Take care to

a. The engine uses an aluminum alloy piston

remove all carbon deposits from the top of the

with two compression rings and one oil control ring.

piston and from the piston ring grooves.

Pistons and rings are available in standard sizes

c. Inspect the piston for cracks, wear, and other

and in 0.010-, 0.020-, and 0.030-inch oversizes.

damage. Refer to table 4-1 for pertinent piston

The oversized pistons and rings accommodate

dimensions. Use new rings when checking piston

rebored cylinder bores. Piston pins are available in

ring side clearance.

standard sizes and in 0.005- and 0.010-inch

d. Check the piston pin for scoring, signs of

oversizes.

overheating, wear, and other damage. Check the fit

b. The connecting rod and connecting rod cap

of the piston pin in the piston and in the connecting

make up a matched set and should not be replaced

rod. Piston pin fit must be within the limits in-

individually or intermixed with the parts of another

dicated in table 4-1.

engine. The connecting rod bearing cap is fitted

e. Inspect the connecting rod for cracks,

with a dipper that dips into the engine oil in the oil

twisting, damaged threads, worn crankpin bearing

pan, thereby causing splashing which lubricates the

surf aces, and worn or scored piston pin bore. Check

working parts of the engine.

that connecting rod and cap dimensions are within

4-8. Removal and Disassembly

limits indicated in table 4-1.

a. Disassembly the pump from the engine (para

Note. Before checking piston ring end gap by inserting

3-2).

the new piston rings in the cylinder bore, check that the

b. Remove the cylinder head from the engine

cylinder bore is within the required tolerances as directed in

paragraph 4-21. Rebore the cylinder if it is scored or damaged

(para 4-4).

and use oversized piston and rings.

c. Remove the oil pan drain plug (14, fig. 4-1)

4-10. Reassembly and Installation

and drain the oil into a suitable container. Remove

the four assembled washer screws (15) that secure

Reassemble and install the piston and connecting

the oil pan (16) to the engine crankcase; remove

rod as shown in figure 4-1, items 18 through 28.

the oil pan and gasket (17).

Note the following:

d. Bend up the tangs on the lock (20) and

a. When installing the piston rings on the

remove the cap screws (18) and flat washers (19)

pistons, use a standard piston ring expander and

that secure the connecting rod bearing cap (21) to

install the bottom ring first. Several types of

the connecting rod.

production and service rings are available. If the

e. Use a hammer handle or similar device to

cylinder bore has not been rebored, use standard

push the piston (27) and assembled connecting rod

size "service" rings which incorporate an expander.

(28 ) out through the top of the cylinder block.

If the cylinder has been newly rebored, use

f. Remove and discard the piston rings (22, 23,

"production"-type oversized replacement rings. If

and 24).

the engine has been in--service for a considerable

g. Remove and discard the piston pin retainer

length of time since the last reboring, use oversized

ring (25). Press the piston pin (26) from the piston

"service" -type rings, Refer to figure 4-4 for proper

(27) to disengage the piston from the connecting

ring installation information. Stagger the gaps of

rod (28).

the three rings so that they are not in line.

4-9. Cleaning and Inspection

a. Piston rings and piston pin retainer rings

should not be reused. Discard and replace.

4-4