Figure 6-25. Cleaning Ring Grooves.

TM 10-3930-633-34

the piston pin holes, can be caused by a bent

connecting rod. Replace pistons that show signs

of excessive wear, wavy ring lands or fractures, or

damage from detonation or pre-ignition.

(3) Check the piston cylinder bore clearance

by measuring the piston and bore diameters.

Refer to table 6-5 for the proper clearance. Refer

to paragraph 6-23 for the bore measurement

procedure. Measure the OD of the piston with

micrometers at the centerline of the piston bore

and at 90 degrees to the pin bore axis. Check the

ring side clearance following the procedure under

Fitting Piston Rings in this section.

(4) Replace piston pins showing signs of

Figure 6-25. Cleaning Ring Grooves.

fracture, etching or wear. Check the piston pin fit

b. Inspection.

in the piston and rod. Refer to table 6-5.

(1) Carefully inspect the pistons for fractures

(5) Check the OD of the piston pin and the ID

of the pin bore in the piston. Replace any piston

at the ring lands, skirts, and pin bosses, and for

pin or piston that is not within specifications

scuffed, rough, or cored skirts. If the lower inner

(table 6-5).

portion of the ring grooves have high steps,

replace the piston. The step will interfere with

(6) Replace all rings that are scored, chipped

ring operation and cause excessive ring side

or cracked. Check the end gap and side clearance.

clearance.

It is good practice to always install new rings

when overhauling an engine. Rings should not be

(2) Spongy, eroded areas near the edge of the

piston top are usually caused by detonation or

transferred from one piston to another regardless

pre-ignition. A shiny surface on the thrust surface

of mileage.

of the piston, offset from the centerline between

Table 6-5. Piston, Pin, and Ring Fits

ITEM

INCHES

PISTON

. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .

Diameter

(red

code)....

. . . . . . . . . . . . . ...3.9984-3.9990

Diameter

(blue

code)

.3.9996-4.0002

. . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . .

Diameter

(0.003

oversize).

. . . . ...4.0008-4.0014

. . . . . .. . . . . . . . .. . . . . . . . . . . .. .

Piston-to-bore

clearance.

.0.0014-0.0022

. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . ..

bore

diameter.

.0.9753

-0.9756

Ring groove width:

. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .

Top.

.0.0800

-0.0810

. . . . . . . . . . . . . . . . . . . .. . . . . ..

Bottom.

0.0800-0.0810

............................. . . .

Oil

ring.

..0.1880-0.1890

PISTON

3.150-3.170

Length

........

. . . .. . . . . .

. . . . . . . 0.9750-0.9753

Diameter

(std).

Diameter (.001). . . . . . . . . . .

..........

. . . . . . . . 0.9760-0.9763

(.002). . . . . . . .

. . .. . . . . . .

. . . . . . . 0,9770-0.9773

Diameter

C l e a r a n c e ( i n p i s t o n ) . . . . . . . .. . .

.. .

............

. . . . . . . . . 0.0002-0.0004

Clearance (in rod bushing). . . . . . . .

...

.............

. . .. . . . . . . . .i n t e r f e r e n c e

PISTON

RINGS

. . . . . . . .. . . . .. . . . . . . . .. . . . . . . .. . . . 0 . 0 7 7 0 - 0 . 0 7 8 0

Width

(compression

rings).

. . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . 0 . 0 0 2 - 0 . 0 0 4

Side clearance (topping).

S i d e c l e a r a n c e ( b o t t o m r i n g ) . . . . . . . . .. . . .. . . . . . . . . ... .

. . . . . . . . . .. 0 . 0 0 2 5 - 0 . 0 0 4 0

S i d e c l e a r a n c e ( o i l r i n g ) . . . . . . . . . . . . ... . . . . .. . . . . . . . . . . . . . . . . .. . . snug fit

R i n g g a p ( t o p r i n g ) . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . 0 . 0 1 0 - 0 . 0 2 0

R i n g g a p ( b o t t o m r i n g ) . . . . . . . . . . . . ., .. . . . . . . . . .. .. . . . . . 0 . 0 1 0 - 0 . 0 2 0

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.015-0.055

Ring gap (oil ring).

6-24