Figure 3-13. Rebushing connecting rods.

Figure 3-13. Rebushing connecting rods.

c. Cleaning.

(1) Soak pistons in solvent and remove carbon

Figure 3-14. Testing piston fit in cylinder.

from the tops and from ring grooves of the pistons.

(2) Using a drill or probe of the proper size, clean

f. Piston Ring Installation.

carbon from the oil return holes in the oil ring grooves.

(1) Always install new piston rings during an

(3) Clean the oil groove passages in the

overhaul of the engine.

Replacement rings are

connecting rods and caps.

available in standard size, in 0.020 inch oversize, and in

d. Piston Fit. Fit the pistons clean and dry. To fit the

0.040 inch oversize. Oversize ring sets are to be used

pistons to the cylinder bore properly, use a spring scale

only in rebored engines.

and a feeler ribbon 1/2 inch wide, 10 inches long, and

(2) To determine whether the ring has the proper

0.003 inch thick (fig. 3-14). Insert the feeler ribbon full

end gap, place it in the cylinder, pushing it about

length into the cylinder bore, about halfway down. Hook

halfway down in the cylinder bore. With the ring square

the spring scale into the end of the feeler ribbon and pull

with the cylinder bore (use a piston to straighten the ring

the feeler ribbon from the cylinder. If the fit is correct,

in the cylinder), measure the gap between the ring ends

the scale should register 5 to 10 pounds pull.

with a feeler gage (fig. 3-15). The ring gap for all rings

e. Piston Pin Fit. Check the fit of the piston pin in the

is 0.010 to 0.020 inch. If the gap is less than the

piston and the connecting rod. The piston pin should be

minimum specified, remove the ring and dress ends with

a thumb-push fit in the piston pin bushing and a palm-

a fine-cut mill file until correct clearance is obtained.

push fit in the piston bosses. Heat the piston to at least

(3) Measure the side clearance of the piston rings

160F in hot water to let the pin enter it readily at final

in the grooves with a feeler gage. The gap should be

assembly. Then pin can be tapped easily through rod

0.002 to 0.004 inch for compression ring. Scraper ring

bushing. If the piston pins or bushings are worn, they

gap is 0.0015 to 0.003 inch and gap for oil rings is 0.001

must be replaced with standard size pins and bushings.

inch to 0.003 inch. If clearance is less than minimum

If the piston bosses are worn, the piston must be

specified, remove ring from piston and rub the ring

replaced.

lightly on piece of fine emery cloth (laid on flat surface)

until proper clearance is obtained.

3-11